Seen it done on a dual core opteron system, in fact I remember a story a while back that compared amd, intel and a mac and in h.264 encoding the amd system won. Weird.
Yeah, last summer I was considering building an AMD X dual core system...with the super PCI express etc. After reading and researching, I just got the iMac 20 Intel dual core. Haven't been disappointed. Yeah, it has crashed a couple times playing video games but so does my XP box. I just turn off the dual monitor and it stays up.
So now, I have my XP box to record TV programs and my wife does email and bills and stuff.