> So am i sticking with 640 and 3gb or should I change to 750 and 4 gb? Will that jump > make much of a difference? or is what i have fine. I dont want to go much higher > price wise
It depends, do you really need a binary copy of every single game ever made in existence? I don't think there has been one person in the history of MAMEkind who has started from the top of the list and completed every single arcade game and clone from start to finish - '88 Games right down to Zzyzzyxx set 2, non-working games, black screen/crash-on-load games and all.
4GB will be fine for Windows 7 to do its useless background crap, although you could technically get away with less than 1GB if you made a 'clean' build of Windows and disabled 99% of the unneeded 'services' it provides (although I am talking from an XP/Vista perspective here).