Yeah, I have a 700MHz and can play a ton of games on it. Obviously there are some that I'd love to play and can't. But there are plenty that are still a blast--think most of the classic 80s games!
Enjoy. Besides, before building your cab, setup the software on your PC and play the games with the keyboard. Although not as fun as playing with a joystick, you'll at least find out WHICH games will work. Then you can design your control panel around those.
Alternatively, setup a desktop control panel to play the games, then build your cabinet second.
Enjoy.