Because of the introduction of SATA, IDE drive prices have been falling, so I'd get whatever cheapest drive you can find. Here in the US you can get 120G 7200rpm 8M cache for about $60: that's overkill for any but the most demanding MAME machine out there.
Even the size should not matter much, it's hard to find anything under 40G these days, and that's a lot of ROMs, hard-drive images and other artwork!