Well the cheapest 256mb X1950pro costs 25 bucks more than the 8600GT
Newegg.com - JetWay X19PR-ED-256M Radeon X1950PRO 256MB 256-bit GDDR3 PCI Express x16 CrossFire Supported Video Card - Retail
The 7900GS costs 20 bucks more than the 8600GT
Newegg.com - PNY VCG7900SXPB GeForce 7900GS 256MB 256-bit GDDR3 PCI Express x16 SLI Supported Video Card - Retail
8600GT
Newegg.com - EVGA 256-P2-N751-TR GeForce 8600GT 256MB 128-bit GDDR3 PCI Express x16 SLI Supported Video Card - Retail
This is the cheapest 8600GT if you include MIR
Newegg.com - Leadtek PX8600GT 256MB Standard GeForce 8600GT 256MB 128-bit GDDR3 PCI Express x16 SLI Supported Video Card - Retail
Edit: And don't forget than X1950Pro you posted is underclocked. X1950Pro at stock have 1380MHz memory speed, but the one you posted have only 1200MHz memory speed
Video Card Comparison - GPUReview.com