Well, they are all PCI Express 8600 GT's..... different prices, the first 3 are 256MB cards, last 2 are 512MB cards....
All have the same max resolution, same memory bandwidth, the core & memory clocks are different on each card.
See the detailed specs for other differences, not going to list them here, because you already have the info right there in the link you provided....
XFX GeForce 8600 GT XXX Video Card - 256MB BFG GeForce 8600 GT OC ThermoIntelligence Video Card - 256MB BFG GeForce 8600 GT OC (OverClocked) Video Card - 256MB Sparkle GeForce 8600 GT Video Card - 512MB EVGA GeForce 8600 GT Video Card - 512MB