If you're looking for cheap and fast, and don't care much for future upgrade paths, you can get a cheap Socket 775 motherboard, some DDR2 ram, a Core2Duo processor, and a video card. I don't exactly have a 'performance' motherboard, but it does support 775, DDR3 ram, and PCIe v2 16X video cards.
I put together a decent (yet cheap) MB, Processor, Video, and RAM which is very similar to what I have now for about $350... you can save about $50 or so going with a cheaper video card... I've heard the 9500 series Nvidia cards do halfway decent. Keep in mind that you should be able to reuse (as long as it's not proprietary) the case, optical drive(s), hard drive(s), and maybe the power supply, but you'll need to check the specs with whatever video card you decide to use:
Motherboard (775 Economy):
http://www.newegg.com/Product/Product.aspx?Item=N82E16813138142Processor (E7500... better and cheaper than the E7400 which is what I use):
http://www.newegg.com/Product/Product.aspx?Item=N82E16819115056Memory (2X2GB DDR2-800... wow DDR2 memory has really jumped up in price):
http://www.newegg.com/Product/Product.aspx?Item=N82E16820145238Video (9800GTX+... very capable card.. possibly the best bang for the buck out there right now, although there are better):
http://www.newegg.com/Product/Product.aspx?Item=N82E16814133246I think this is going to about par for a $300-400 upgrade. Yes you can find upgrade options for under $300, but you won't get as much 'value' out of it. If you want a system that is state of the art, or that you can upgrade later on, it's obviously going to cost quite a bit more...
Anyways, the above recommendations is about the same as what I have (I have a slower processor, but faster ram)... and with the game fully maxed, I never see below 50FPS. With shadows MAYBE 45.