FPS the same @ 1920x1200 and 1280x1024

[Spec7re]

I recently did some tweaking to try and up my FPS, and my monitor is a 16:10 widescreen, so the only selectable resolution was 1920x1200, with muted PP, tril off, and bloom off, i was getting 25-50 FPS on all maps, and now I can't change resolutions without crashing the game (possibly because of nHancer), so i opened the UTEngine.ini and dropped the resolution to 1600x1200, I infact had lower FPS than 1920x1200, so I went back and dropped it down again to 1280x1024 and I am getting the exact same FPS than before, any ideas on how I could up my FPS in general (on 1920x1200 even)?

edit:
2x GeForce 7900 Go SLI
2gb DDR RAM
AMD Turion MD40 @ 2.2ghz
Windows XP Media Center Edition