Because: 2560x1600 is HUGE
You are probably getting what you should get o.O
Sell your 2900XTs and buy 2x 8800GTs or 2x 8800 GTS <b>SSC Editions [important they are these ones!]</b>, probably the latter is better for you.
Or 2x 8800 Ultra.
Or..if you lower the res to about 1440*900 you can play all max, no AA
Oh and disable AA if you have it enabled
Don't expect anything more than 30 FPS even with overclocked 8800 Ultras...
I run Crysis with 1x 8800 GTS massively OCd at about 35FPS all max no AA at 1280x1024, so your 2x XTs are about 150% of the performance of mine, and so you should be able to run the game at my settings at about 50fps.
Since you are running at 3.125x the resolution. you should be running at 50/3.125 fps which is 16. If you are running DX10 max on Vista expect about 10.
Sorry to kick you in the balls. But seriously, SELL YOUR 2900XTs AND GET 2X 8800GT/8800GTS SSC EDITION AND THEN OVERCLOCK THEM. THEN EXPECT ABOUT 25 MAX ON XP.
I don't think you quite get how BIG your resolution is...
Hope that helps.