A 9800 Pro at $174 is hands down the most bang for the buck you can get. Most places have them in the $205 - $215 range. If you have a fast computer (AMD 2400+, P4 2.4 +) the 9800 Pro is the ATi card to have. If your computer is older, it may bottleneck the card in which case the 9600 XT or Pro would be a better choice. nVidia makes some good cards too. I just don't have any experience with them.
Online prices have the 9800 Pro @ $205, The 9600 XT @ $159, and the 9600 Pro @ $139 all of which represent pretty good bang for the buck. The Ati SE (and nVidia MX)cards are marketing gimicks. You pay $13 more for a 9800 SE than a 9600 Pro but they perform about the same. The 9600 SE barely outperforms the 9200 again for more money.
The Omega drivers will not improve frame rates and may be slightly slower (based on personal testing a few versions back). They are supposed to improve image quality. I run both Cat and Omega drivers on my computers and can't say I can see a difference even with my bifocals. Younger eyes may do better. Omega drivers come with RadLinker, a good overclocking utility.
If higher frame rates are what you're after, make sure Antialiasing (AA) and Anisotropic Filtering (AF) are set to application preference.
Contrary to what MrLars says, you can't get 9800 Pro speed out of a 9600 XT. The 9800 Pro has twice the buswidth and twice the pixel pipes of the 9600 XT and no amount of overclocking will overcome that. (I have both and have benched them stock and overclocked).
My experience with the default settings in AH2 is they work ok for air to air combat but are unsuitable for bombing and attacking. You have to set the sliders to significantly more detail and greater range if you want to see ack positions and small buildings. Frame rates will be quite a bit lower near the ground with higher quality settings.