Here are the specifications of the computer that powers my cabinet.  It runs all games emulated by M.A.M.E. at full speed except some of the newest ones like Cruisin' U.S.A., which require a 3GHz or higher processor.  It should be noted that I also use this as my main PC (games, multimedia, and occassionally work). Depending on what you want to use your PC for, you may not need one like this.