I have the macbook pro with the radeon x1600 card and can play up to BV in OSX and have played Seasons in XP. It does get hot when playing graphic intensive programs.
I play though on a ppc iMac with the radeon 9600. Not the best card, but I play up to BV with it. With just a small amount of cc...I have no patience to wait for the load time more than anything.
Basically by using Bootcamp your mac becomes a pc. *gasp* Horrendous I know....
Now the new iMacs are also small, but I don't think overheating is too much of an issue. You could probably get the 24" iMac for the same as the Macbook Pro. Some people have been having issues with the new video card on the intel iMacs, with up to Seasons, and Aspyr is telling them to buy BV as it fixes it. But on the windows side, thats going to be different. I don't know how that video card relates on the windows side.