Woot! Another 5800U devotee!
Man, what are the rest of your system specs, and have you done any tweaking to the config? My 10x7 medium quality timedemo nets me about 37 fps (the 8x6 high quality timedemo gives me a higher score, but it hitches a bit every now and again). Granted, I'm a little old on the rest of the stuff, but that seems like a rather large difference!
I have a A643000+/MSI K8T Neo/1GB Corsair PC3200, that is the difference. You're cpu limited for Doom3, no offense intended.