yes, the question is asked a lot and there is not "right" answer. Your hardware requirements depend largely on which games you're willing to give up support for.
A large percentage of game will run very well on old hardware, especially with older versions of MAME, like paigeoliver suggests.
A number of games will not play even with the latest Pentium 4 Extreme Edition, like a number of the CHD games.
The same principles apply to "what video card", "what harddrive size", "how much RAM", ...