edit - Wow... the prices on Amazon seem a bit high... you should purchase from eBay. You can "buy it now" an X7900 on eBay for about $140ish or a T9500 for about $160ish.
Also, view a thread I started on another site... just ran through all the good/bad with x7900, x9000 and T9500.
I got lucky and recently grabbed two X9000s off ebay for pretty good prices (one for $207 shipped and another one for $220 shipped)
Basically the performance goes like this though...
Now the T9300/T9500 can show around equal to a X7900 in benchmarks, but in real world use the T9500/T9300 will generally do better. The biggest plus to the X7900 or even X7800 are of course the unlocked multi... but chances are the best OC you'll get is around 400Mhz and I'm not certain that even with that OC it would be a "better" deal vs a T9500 or maybe even T9300.
With my retail X9000 I can run it at 3.4Ghz w/VID @ 1.2875 (a 600Mhz OC)... my "ES" X9000 though tops off at 3.2Ghz because it requires a higher VID and I run into overheat issues with the stock cooling of the 6860FX. If I was better at modding, I could see getting another 200Mhz of of each chip as the only reason I can't go further is the stock cooling. I'm using MX-4 thermal paste and my retail chip at 3.4Ghz hovers around 79c on full load with Orthos.
Keep in mind that a X7900 will run a little hotter (maybe 5c?) than my X9000 so I would assume a 200-400Mhz OC would be about max.
FYI - The max stock OC (for X9000/X7900) via the 6860FX BIOS is 3.0Ghz & 1.2v VID, but if you use the program ThrottleStop you can OC a bit further or undervolt if you want to keep it at stock 2.8Ghz or 3.0Ghz.Edited by teknomedic - 5/19/11 at 1:49pm