Pokerroom runs really well IMO (it's a Java client you launch from their web site). Pacific (888.com) is the same kind of thing, but is not as smooth, and you can only play one table at a time.

I haven't tried GamesGrid yet, but I probably will, because I've got to support any reputable site that makes a Mac client. (There's a rumor that Full Tilt is working on one too.)

I wouldn't recommend Virtual PC for anything less than a dual G5 - I've got one and it still runs fairly slowly. In general, only the most well-engineered sites are usable with VPC, like PokerStars, Ultimate Bet and the Party skins. Multitabling bogs everything down further, and can even lock you up for a few seconds at a time.

I'd recommend getting a cheap Windows laptop for playing poker if you want to go much further than Pokerroom, GamesGrid and Pacific, but that should depend on where you are at poker. If you're just getting started, play the micro- or nanolimits at those sites and wait until you're good enough to justify the expense.