Can i overclock the 2600 Pro to like Xt speeds?
I doubt it,
but as other said, I will really advice you to not buy 2600pro or 8600GT.
9600GT, 8800GS, 9600 GSO, Radeon HD3850 (in this order) or something in that range will be probably more expensive but such small amount more will give you 2-3 x performance so better save up $20-30 more and buy one of those. you can save money buying their 256MB parts as it will be less expensive and you can game at lower resolutions with same performance. (1024x768 or 1280x1024 max)
If you cant afford one of those buy a 8600GTS at least , its 2x faster than 8600GT in some cases and is like $10 more if you find deals