[Answer]-Alternative to global variables in Django?

Thank you @PeterRowell for pointing me in the right direction. I used django sessions to store the filters, javascript to send a request to the server to update the filters when they load the form and to delete them when they leave the form, some view functions to handle those request, added a function to the model (with the request as a parameter) to update the filters per what parts where already saved, and override the changelist_view function of the PartAdmin to use the filters in request.session to modify the query string. That was a lot of code in a lot of different files, but here are some of the highlights that will hopefully help someone looking for a solution like this:

All the models posted in the question stayed pretty much the same.

The views for the PC:

def update_filters(request):
    try:
        # get POST data
        url = request.POST['form_url']
        id = url.split('/')[-2:-1][0]
        system_type = url.split('/')[-3:-2][0]

        if id is not "new":
            system_content_type = ContentType.objects.get(app_label="systems", model=system_type.rstrip('s'))
            system = system_content_type.get_object_for_this_type(id=id)
            system.set_filters(request)
        else:
            request.session['filters'] = ''
        return HttpResponse("Filters where updated.")
    except:
        return HttpResponse("Select a part and click 'Save and continue' to set the filters.")


def delete_filters(request):
    request.session['filters'] = ''
    return HttpResponse("Filters where deleted.")

Here is the javascript that was placed in the change_form.html (added it via the extra_context param in PcAdmin add_view and change_view functions)

    function post_to_url(path, params, method) {
        method = method || "post"; // Set method to post by default, if not specified.

        // The rest of this code assumes you are not using a library.
        // It can be made less wordy if you use one.
        var form = document.createElement("form");
        form.setAttribute("method", method);
        form.setAttribute("action", path);

        for(var key in params) {
            if(params.hasOwnProperty(key)) {
                var hiddenField = document.createElement("input");
                hiddenField.setAttribute("type", "hidden");
                hiddenField.setAttribute("name", key);
                hiddenField.setAttribute("value", params[key]);

                form.appendChild(hiddenField);
             }
        }

        var frame = document.createElement("iframe");
        frame.name="hidden-form";
        form.target="hidden-form";
        document.body.appendChild(form);
        document.body.appendChild(frame);
        form.submit();
    }

    // when they load the page, set the filters
    $(document).ready(function(){
        post_to_url("/quotegenerator/systems/update_filters/",{'form_url': document.URL});
    });

    // when they exit, delete the filter cookie
    window.onbeforeunload = function() {
        post_to_url("/quotegenerator/systems/delete_filters/", {});
    }

And finally, the function that was added to PartAdmin:

    def set_filters(self, request):
            # query and get the parts
            try:
                    cpu = self.cpuChoices.get()
            except:
                    cpu = False
            try:
                    mobo = self.motherBoardChoices.get()
            except:
                    mobo = False
            try:
                    mem = self.memoryChoices.get()
            except:
                    mem = False
            try:
                    hdd = self.hardDriveChoices.get()
            except:
                    hdd = False

           # for each combo of parts, figure out whats required
            # no parts at all
            if not (mobo or cpu or mem or hdd):
                    request.session['filters'] = ''
            # mobo only
            elif mobo and not (cpu or mem or hdd):
                    request.session['filters'] = 'socket='+mobo.socket
            # cpu only
            elif cpu and not (mobo or mem or hdd):
                    request.session['filters'] = 'socket='+cpu.socket
            # memory only
            elif mem and not (mobo or cpu or hdd):
                    request.session['filters'] = 'memType='+mem.memType
            # hard drive only
            elif hdd and not (mobo or cpu or mem):
                    request.session['filters'] = 'interface='+hdd.interface
            # mobo and cpu
            elif mobo and cpu and not (mem or hdd):
                    request.session['filters'] = 'memType='+mobo.memType+'&interface='+mobo.interface
            # mobo and memory
            elif mobo and mem and not (cpu or hdd):
                    request.session['filters'] = 'socket='+mobo.socket+'&interface='+mobo.interface
            # mobo and hd
            elif mobo and hdd and not (cpu or mem):
                    request.session['filters'] = 'socket='+mobo.socket+'&memType='+mobo.memType
            # cpu and mem
            elif cpu and mem and not (mobo or hdd):
                    request.session['filters'] = 'socket='+cpu.socket+'&memType='+mem.memType
            # cpu and hd
            elif cpu and hdd and not (mobo or mem):
                    request.session['filters'] = 'socket='+cpu.socket+'&interface='+hdd.interface
            # memory and hd
            elif mem and hdd and not (mobo or cpu):
                    request.session['filters'] = 'memType='+mem.memType+'&interface='+hdd.interface
            # mobo cpu and mem
            elif cpu and mobo and mem and not hdd:
                    request.session['filters'] = 'interface='+mobo.interface
            # mobo cpu and hd
            elif mobo and cpu and hdd and not mem:
                    request.session['filters'] = 'memType='+mobo.memType
            # mobo hd and mem
            elif mobo and mem and hdd and not cpu:
                    request.session['filters'] = 'socket='+mobo.socket
            # cpu mem and hd
            elif cpu and mem and hdd and not mobo:
                    request.session['filters'] = 'socket='+cpu.socket+'&memType='+mem.memType+'&interface='+hdd.interface
            # all parts
            else:
                    request.session['filters'] = ''

Oh, and the changelist_view function of PartAdmin did change a little:

def changelist_view(self, request, extra_context=None):
    if ('filters' in request.session):
        q = request.GET.copy()
        for filter in request.session['filters'].split('&'):
            key, value = urllib.splitvalue(filter)
            # check if the request does not already use the filter
            # and that the model has the attribute to filter for
            if (not request.REQUEST.has_key(key)) and (key in self.list_filter):
                q[key] = value
        request.GET = q
        request.META['QUERY_STRING'] = request.GET.urlencode()
    return super(PartAdmin,self).changelist_view(request, extra_context=extra_context)