If you were willing to build a computer yourself, and knowing that you already have a new video card, as an example, here's a cheap system that you could build around it for just over $250 that would be a vast improvement over what you have. You'd have to use your current keyboard, mouse, monitor and speakers. I included both hard drive and optical drives to upgrade to SATA although your old IDE drives would work and save you another $62 bringing the total under $200. This doesn't include the operating system.
Update LOGISYS Computer Area 51 CS51WBK Black Steel ATX Mid Tower Computer Case 450W Power Supply - Retail
Model #:CS51WBK
Item #:N82E16811148030
Return Policy:Standard Return Policy
In Stock
$39.99 $39.99
Update AMD Athlon X2 BE-2400 Brisbane 2.3GHz Socket AM2 Dual-Core Processor Model ADH2400IAA5DO - OEM
Model #:ADH2400IAA5DO
Item #:N82E16819103215
Return Policy:Processors (CPUs) Return Policy
In Stock
$39.99 $39.99
Update ASUS M2N-E AM2 NVIDIA nForce 570 Ultra MCP ATX AMD Motherboard - Retail
Model #:M2N-E
Item #:N82E16813131022
Return Policy:Limited 30-Day Return Policy
In Stock
$84.99 $84.99
Update Western Digital Caviar Blue WD800JD 80GB 7200 RPM SATA 3.0Gb/s Hard Drive - OEM
Model #:WD800JD
Item #:N82E16822135106
Return Policy:Limited 30-Day Return Policy
In Stock
$38.99 $38.99
Update LITE-ON 20X DVD±R DVD Burner Black SATA Model iHAS120-04 - OEM
Model #:iHAS120-04
Item #:N82E16827106263
Return Policy:Limited 30-Day Return Policy
In Stock
$22.99 $22.99
2x Update Kingston 1GB 240-Pin DDR2 SDRAM DDR2 800 (PC2 6400) Desktop Memory Model KVR800D2/1GR - Retail
Model #:KVR800D2/1GR
Item #:N82E16820134487
Return Policy:Memory (Modules, USB) Return Policy
In Stock
$20.49 -$6.00 Save $28.98
Subtotal: $255.93