I wouldn't bother with a Soundblaster to be honest...so many of the onboard Intel soundchips are so good now that you won't feel the difference. A lot of onboard chips even offer 5.1/7.1 at HiDef levels. It's a matter of preference of course but I believe the one to avoid is the Audigy.
Any GeForce will run Compiz to some degree; I previously ran it on a 6200 without a glitch. You could buy a base level 8xxx series and have great performance if you aren't gaming much. Don't forget though that ID release their games for Linux and there are great games like Tremulous and Urban Terror that are native and free.
My own rig was a beast when I got it though it could be had for a few hundred euro now. It's a Q6600, nVidia 8600GT and an ASUS board with on board nVidia sound. I have 2GB of RAM and it's never had a stutter. Well maybe once but that's another story
