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