My observations on AH2 FPS and related issues:
1. FPS alone is inadequate to tell if your machine can handle AH2. FPS really only affects air-to-air fighting. It's possible to turn down graphics settings to get fairly high FPS even on "old" machines, but this is at the cost of making ground objects like buildings become invisible beyond 1 mile or so. This makes buffing and jabo ops totally impossible because you don't have time to aim your bombs once you finally get close enough to see a target.
So while you MIGHT be able to tweak things so a "dinosaur" system can fight half-way acceptably air-to-air, that's ALL you'll be able to do. To fully participate in all aspects of AH2, you need a system that can give you high FPS with all the details on, so you can spot buildings far enough away to have time to aim bombs at them. And at present, this pretty much means the top-of-the-line system.
2. That said, I need to elaborate on "old" and "dinosaur" systems, because in AH2 (so far--beta 18), these terms mean different hardware than in other games. As things stand now, everything less than 2GHz is a "dinosaur", unable to cope at all with any aspect of AH2, even with the details turned down as far as they'll go. 2.2-2.6GHz machines are "old", as in barely adequate even with sacrifices like no jabo ops due to invisible buildings.
The following system (mine) is an example of a dinosaur. Even with all graphics options turned all the way down, it is impossible for this machine to get an acceptable air-to-air FPS when there are more than 2 other planes near me:
P4/1.7GHz
512MB RAM
ATI Radeon 9600 Pro 128MB
3. I say the above fairly confidently. I seem to be one of the few who have seen more than 2 or 3 other planes online in AH2 so far. 2 weeks ago, my squad did our squadnight there and had furballs of up to 10 planes at a time, with an average of 8. You know, what you see in AH1 at a MINOR battle. Nobody had acceptable FPS, even guys with 2+ GHz systems, although they said they could live with it (and of course no jabo ops for them).
4. The FPS you see by yourself offline is quite misleading. With all settings minimized, my system above can get 50-60 FPS offline. It can do the same online when I'm by myself. However, once just a couple of other planes get close, FPS plummets. When I have 8-10 other planes in icon range, my FPS gets into the low 20s. When I get within several hundred yards of 1 plane, I'm in the low teens, and when I shoot or get shot at, it becomes a slideshow from the muzzle flashes alone, even without tracers on.
5. At present, GeForce cards have a big advantage over Radeon cards. When AH2 first came out, I had a 3-yr old 64MB GeForce3 and 256MB system RAM. Changing the vidcard to the brand new Radeon and doubling my system RAM resulted in a 20% reduction in my FPS. And in general from reading posts here, it seems like AH2 just doesn't like Radeons. Whether this will ever get addressed is unknown. But if I was going to buy a new vidcard for AH2 today (actually, I did again last week, along with a whole new 3.4GHz system), I'd definitely go Nvidia instead of ATI.