I just got my gf a Shuttle SN95G5. I put an Athlon64 3000+ chip in it.. with the variable speed fan, and frequency scaling.. it runs at 1.0ghz most of the time.

It runs fairly cool, and quiet enough that the fridge is louder.

I wanted to use a Nvidia FX5200 card with no fan.. but I had trouble with the DVI output on her Samsung 730b.

Opterons are only usefull in dual-CPU setups, but I highly reccomend the socket 939 setup. having on-die memory controler is very nice.