Unfortunately I play FEAR, Oblivion, Gothic, Spellforce, AOK. Games in the FPS and RTS genre.
Yeah those games will play fine at 2560x1600, I used to do that before I got my 8800gtx. With FEAR however you will have to scale down to 1920x1200 or 1920x1600 as there seems to be an issue with 2560x1600 with very poor performance and a small lighting issue in a couple of places. I am talking about max settings however, if you use medium you will be fine.
Oblivion is fine with an XT, but I would suggest using your overdrive feature to get XTX speeds ,as it's very easy and helps alot. With the Xt at XTX levels I was playing with max distance sliders, low shadows, hdr and 2xAA. It was playble and looked great.
Gothic 3 works great with max settings.
Alot of other games worked very well with 2560x1600 and the X1900XT(X). Including Tomb Raider: Legends, Hitman: BM, Fablle etc.
One thing though you will need a very pwoerful cpu to get the most out of Oblivion and Gothic 3, especially at 2560x1600. If you have an Athlon (dual core)64 chip reckon on pushing your cpu to around 2.6ghz at least to get the most out of it. You will also have to install that dual core driver .