To play CMBB/CMBO, right now, the GeForce 4 Ti4200 128 mb is probably the best bang for the buck. It is nearly as fast as a Ti4600 for about two-thirds of the price and it can handle all of CMBB/CMBO's graphics. In CMBB, the CPU is more of a bottleneck than the video card.
I have a Radeon 8500 and even though I find it is a great card, with very impressive image quality, it was not designed for CMBB/CMBO. The 8500(as well as the 9700pro) cannot render fog because of the way it is implemented in the game and does not support 16 bit FSAA.
Kump, I am surprised you are having problems. I have a 1.2ghz athlon Tbird, 512 mb sdram, XP pro, Radeon 8500. I play CMBB at 1280x960 with max graphics and even big scenarios like "Kalinin Raid" are very smooth. A "HUGE" scenario like "On to the Volga" is a slide show, but from what I read in other posts, even users with 2.2ghz Pentium 4's are having problems running those scenarios.
On your particular choice, right now the 2.8 P4 is about twice the price of the 2.53 P4 for only about a 10-15% improvement and NVIDIA is supposed to release the NV30 which will replace the Ti4600 next month.
But I am in the same boat myself, trying to decide whether I should upgrade now or wait until next year.