Hm. I haven't got a solution, but I can offer a comparison.
2.6 GHz P4, 1 GB RAM, ATI Radeon 9600TX (NOT XT, about the power of a 9600Pro), Win2k, DX9.0b, Cat 4.5 (Omega drivers).
On 1280x960 with 2x AA and 4x AF I get 20FPS on normal, 12 on high, 8 on very high and 5 on too high.
I'm not sure what could cause this problem. A friend of mine had the same very low FPS as you, until he upgraded to newer Omega drivers. Since you already tried that...
