You can build a PC with virtually any part available today and max out PS2 games, as long as your emulator is good. I doubt you'll need that much RAM for a PS2 game, but if you do plan on playing the occasional PC game then 3GB minimum would be a good place to start.
The MacMini has a 9400M graphics card. You'd be hard-pressed to find anything that WOULDN'T beat that card.
You could put together a considerably faster rig for $800 if you picked out parts closer to your purchase-date. It's difficult to recommend anything today. Ask again in June.