I am one of the few mac users here on FTR. I mainly play on Pokerroom, as their java client is mac compatible. I also play on Partypoker and Ladbrokes, using VirtualPC. I have Windows XP, NT4, Hungarian version of 98 (don't ask) and a bunch of different linux distros insatalled. Mainly use XP for poker stuff as it is generally more compatible.
In general, any mac you buy today wil be able to run VirtualPC with some flavour of windows fast enough to cope with pretty much any poker software you throw at it, as it is not a graphics intense thing (which is the main failing, as the emulator cant really utilise the power of the graphics card).
There is also one big advantadge. You can make up a master image before ever connecting to the net with a basic ammount of sotware installed and then duplicate it at will, so you will always have a clean image to start from. This means when the 5 billion virii out to get you get past you Anti-Virus software, you only have about 5 keypresses and 2 minutes for a brand new clean install.

hope this helps