I installed PBX in a Flash on an Intel D945GCLF2 mini ITX computer that I used to use as a mini XP box - the performance with XP was only OK, but running FreePBX/Asterisk with CentOS on it is great.
It can handle multiple incoming and outgoing calls without breaking a sweat (I tested 3 concurrent connections so far with processor load under 50%.)
I'm running it "headless", with just an ethernet and power cable plugged in.
To the left is picture of the new home PBX.
I've loaded up Ubuntu on the PC I was using for the PBX - everybody wins!