Like I said: poorly optomised, lack of driver support. It's not your hardware that's the problem.
For example I find that if I load a quicksave in-game, my FPS drops to the 20 FPS region. Load it immediatley again, I'm back @ 50 FPS or whatever. Weird.
AMD X2 4400+ Toledo 1MB/Asus A8N-SLI Deluxe/2GB Kingston HyperX/SLI Gigabyte 7800 GTX 256MB/Creative SB Audigy 2 ZS Plat w/ Creative Inspire 6.1 speakers /180GB Hitachi PATA/19\" AG Neovo F-419 LCD/OCZ PowerStream 600W PSU/WinXP Home SP2