Ok, here's what I scrounged up for around $800 or less. This system is pretty good for everything. Not the best gamer computer, but it will do nicely for all games. Now you say you have a video card already so I'm not including that. Just make sure that your card is a PCI-e compatible graphics board:
AMD Athlon 64 3700+ San Diego 1GHz FSB 1MB L2 Cache Socket 939 Processor - Retail
http://www.newegg.com/Product/Product.asp?Item=N82E16819103539
MSI K8N Neo4 Platinum Socket 939 NVIDIA nForce4 Ultra ATX AMD Motherboard - Retail
http://www.newegg.com/Product/Product.asp?Item=N82E16813130484
CORSAIR XMS 1GB (2 x 512MB) 184-Pin DDR SDRAM Unbuffered DDR 400 (PC 3200) Dual Channel Kit System Memory - Retail
http://www.newegg.com/Product/Product.asp?Item=N82E16820145450
Western Digital Caviar SE WD800JD 80GB 7200 RPM 8MB Cache Serial ATA150 Hard Drive - OEM
http://www.newegg.com/Product/Product.asp?Item=N82E16822135106
ASPIRE X-Gear ATXB5KLW-AL Silver Steel ATX Mid Tower Computer Case 420W Power Supply - Retail
http://www.newegg.com/Product/Product.asp?Item=N82E16811144041
This system will OWN. Pretty good for the price. Damn...San Diego core!!! TOtal cost: $637.25 not including tax but I did calulate shipping already.
If you still want a video card, get this one:
PNY VCG6800GXPB Geforce 6800GT 256MB 256-bit GDDR3 PCI Express x16 Video Card - Retail
http://www.newegg.com/Product/Product.asp?Item=N82E16814133140
Add $274 to that order. I included the rebate for the video card too. It comes with a analog RGB to DVI convertor so don't worry about compatibility. It will work.