It depends on the board, chipset, other bits of the machine and more importantly the game.
Having a seperate graphics card will help, but ive never seen an itx board with an agp slot and im not sure what the performance would be of a PCI card compared to the onboard gpu which usually claim to be agp 2d/3d.
Loadsa memory would also help, you should generally buy the largest compatible sticks you can find.
Now that via are pushing their boards over 1ghz (sure i saw an ad for a dual cpu mini-itx board also) i guess playing Quake 3 wont be so hard, though why bother when theres nethack
daihard: some of them are fanless which makes them rather quiet, which is nice. everything is generally onboard, although most boards are like that now. I have a little itx system acting as a small lan server running linux. Its seriously tiny, with a little tiny power supply to power it and a little tiny cardboard case i made. I guess im just cheap.