My rig was pretty outdated (single core amd, 7200, 4gb memory), 5boxing was "ok" in instances but performance was still very jittery and far from being acceptable for doing anything in PVP...shatt/org were basically inaccessible. Anyway I put together a new rig a few weeks ago:

1333fsb setup
Q9550, P45 Asus board (forget the model), 8gb memory, 64bit vista, 280gtx, 64gb SSD (OS and wow configuration/instance directories are on a standard drive)

No idea if it's an optimal setup or not, just seemed like a good bang for the buck. Anyway i'm totally happy with the new rig, about $1,000 worth of stuff (and my AV box got hooked up with the handme downs!) game runs 100% flawlessly with 5 clients, you can spin the camera anywhere at anytime and theres never any jitters or stuttering. Craziest thing was how fast PIP swapping became. Before it would take ~5 seconds, now its literally instant (using keyclones PIP)