I started out with an old Via C700 (1Ghz x86 processor), and it was too weak for my uses. It stuttered at times with MP3s, and video was not playable at all.
Video was not a killer for me, but I did want it to work. The screen slides in Centrafuse were poor too, and doing NAV and playing MP3s under Windows was too much.
I ended up replacing it with a i3-550 (on-chip video) and have never had issues since. Of course, my system now takes 4-5A just for the PC/screen/devices, so I pay for the power.
If you are like most of us (I suspect), you'll spend a ton of time putting the system together and making it work. My advise is to buy a new, good MiniITX pre-built setup. Believe me, It'll save tons of frustration.