I am running a 9800GT as well and I get better then 4 - 20 FPS. The system is a Dual core AMD @ 2.5Ghz, 3 GB RAM, and the 9800GT 512MB (EVGA model). I get dips from time to time, but manage to enjoy my dogfighting and it looks better then IL2
Granted, most everything is turned way down, but it does run at a playable FPS.
My mother board will support up to Quad-Core AMD chips (I do have the specifics, just not off the top of my head). I run Vista 32-bit so 4 GB RAM is the most I can use anyway. I do plan on getting a 550 at some point, but it can wait till I get a good deal. I only paid $20 for this 9800, so who knows
