I'm not sure if you are looking for a manufacturer or to build. Personally I would choose to build.
I built and amd dual core with an asus mb and gt8600 card. Seems to handle the game quite well. However, there are some issues with the amd dual core and you have to download some fixes and maybe rework the registry a little bit. It may be best to go with an intel chip. I only spent about 600 bucks, but then I already had the sound card, case, cd drives and power supply. The amd chip was about 100 less than the comparable intel.
Hope that gives you a basis to build on.