If you're looking to buy, a uATX (otherwise known as a microATX) will fit in a standard ATX case without any problems.
The setup you had picked out looks pretty decent, the only real thing to note is that you have an X2 processor at the moment, and mame won't really take advantage of dual core processors for most things. That being said, there really aren't any _faster_ single core options for that range so I'd probably just go with that proc.
Might want to look at the package deals, looks like your biostar motherboard has a 10 mail-in, where-as if you use one of the package deals, you'd get 10 bucks right off the top for an ECS motherboard with the same chipset.
Gig of ram should be fine, and HD looked OK. Not a huge ATI video card fan, but that'll do the trick for what you need it for.