First off, you could do a tech-forum search fairly easily and find good gaming machines for under a certain price. See mine in the specs? That ran me about $1200 or $1300 when my processor was pretty new to the market. Now you would want to swap it out for a 3200+ or a 3000+ with a Venice core. The graphics card is still darn good. The motherboard is also good, but DFI just released its Lanparty with AGP and it already had one for PCI-Express. The difference would be to not get an LCD (which costed me a pretty penny) and get a PCI-Express motherboard instead of AGP. Also, a PCI-Express graphics card, probably 6800GT, but wait until the new cards come out now and either get one of those, or watch the prices fall so you can snag the 6800GT cheaper. If you get all that, and not the hard drive or RAM, then you should be around $1000.
How big is your RAM? 512 MB sticks? Is it DDR 400? Is it 184-pin?
AMD Athlon 64 3000+ socket 939 @ 2.3 GHz
MSI K8N Neo2 Platinum motherboard
EVGA GeForce 6800 GT
2 GB DDR 400 (PC 3200) (2 x 1 GB sticks) (Dual Channel)
DVD player and seperate DVD RW drive
80 GB Western Digital Hard Drive
500 watt power supply
Sound Blaster Audigy
Logitech 5.1 surround sound speakers
SAMSUNG SyncMaster 710T LCD monitor
Aspire X Navigator Case