It's playable, but my 3ghz P4 and 9700pro should be able to run it smoothly, and in a few of the larger rooms, it definately was not smooth.
How much video ram? How much computer ram? Crappy hard drives? Other things can lead to this...even maybe the 4.5 drivers you are using.
Athlon XP2800+, 9800pro 128mb, 1gig ram, SATA drives...I'm running it very smoothly at 1280x1024x32. I'm running 4.3 drivers and the Hardware button on the Autorun told me to upgrade, but I ddnt bother...figured I'd see if I have any problems first....which I havent. Just an FYI, DX:IW ran like crap on this same system at 1024x768x32 with Bloom off.