I dont know anything about N-vidia cards, but if the card has an option to turn off "v-sync" in the 3-D set-up, that should double your rates. Also, when was the last time you did a full system check for viruses and spyware? Those can really bog a system down.
Another tip is to download a program called FSAUTOSTART. I dont have the download web site handy, but if you google it using that name you will find it easy. That program shuts down background services on your computer..which frees up rescources.
I run off an Athlon 1800XP (1.53 Ghz) and an ATI 9600 Pro and get max rates in the 80's and 90's..and minimum rates in ground view in the 20's. I read threads about people with a lot better systems and better graphics cards than me having worse frame rates.
All else fails, try doing a fresh re-install of your operating system and load up AH2 right after that (so you dont have to defrag it later). This way, Any unknown viruses or spyware are whiped
Oh, not sure if this will work for you, but in the AH2 graphics setup where you have the option of turning off "transitions" "horizon" and "mip-map". I have found that I get about 5 fps more if I have the Mip-Map
ENABLED.
Last, but surely not least.. SKuzzy has sticky posted "Tips" on some of these forumns on how to get the most out of your system for AH2
In the mean time, you may want to run DXDIAG.EXE (In your direct x folder) and run a report and e-mail it to Skuzzy. He can give you some tips if he sees anything that can be improved.