I think you would be horribly disappointed with the 8400. Tom's Hardware 2007 VGA Charts. *edit* Guess Xzin beat me to it while i was typing this up.

Notice how the 8600 GT performs 3x as well as the 8400, yet there is only a $30 price difference. EVGA 8600GT 256MG: $89.99

Also, I have two friends that have used the shuttles in the past. Both of them had heat issues that made the system unstable. They were fine for non gaming use, but that is not your goal. I purchased my rack mount cases, a 500 watt power supply, and a reliable MB for less than what you will be paying for that little shuttle.