do you want small? if so miniitx.
if you dont mind, atx is cheaper speed for speed...
(a fast miniitx is much more expensive)
other limitations= some atx board are very power hungry, some itx cases dont allow pci card, and so on and so forth.... what kind of space/money/performance do you want? do you want to watch dvds, gps etc all at the same time? do you want a performance system or just a startup? give us more info
2005 Range Rover 4.4 Any ideas for putting a PC in this? :)