You'd get around 40 FPS average with an 8800 GTS 320MB with dips no lower than 26 (higher than that, as again these numbers are for 1280x1024), where as the HD2600 XT will drop to single digits when things get rough at times. That's the main difference -- maintaining playable framerates when the game gets more demanding, not running much higher framerates in the 'average' situation.
Installing the Float 32 modification should improve image quality /and/ speed up the game significantly, however, and I can't find any reviews running Float 32 to give you numbers with it... STALKER is known to be under-optimized to a great extent.
![[H]ard|Forum](/styles/hardforum/xenforo/logo_dark.png){kind=logo}