My P2 233 will run all of the classics like defender, centipede, etc.... think black background coloured sprites.
It actually ran WWF Superstars well enough for my brother and I to have a game. It was really poor framerates though. I wouldn't call it playable for enjoyment, mostly just to show off the cab in progress.
I'm trying to get sorted out to buy a new computer so I can put my 1.7GHz in.
If you stick to way back classics, not much problem. You limit your game list seriously though.