I have the Q9450 and the EVGA gtx280 (which runs just fine, no lockups, maybe the gigabyte version is the problem for momo) and I have 8 gig system ram - runs 5 clients just fine but I do have the backgrounders all turned to minimums.
I am definitely going to upgrade to an i7 setup before year end so I can turn up the view distance on the followers substantially. Right now when I'm flying them high above the ground I am flying by instruments only, since they can't see anything but grey fog. It's only an issue when they drop off follow and i don't notice for a while, or have somehow managed to survive an encounter with just my DK standing and can't rez anyone up.