Hey,
Writing to say that I’ve been animating in After Effects on a Macbook (black 2.16 2gb ram) since the fall using CS3, and while it takes longer than I’d like to render in HD, I’ve made 4 HD animated short films using a mixture of Painter, Photoshop and illustrator art, on tight deadlines without a single problem. It’s been a fantastic little machine. Heat was never a problem for me, and it only got a touch angry, and got really bogged down, if you tried to do too much while it was rendering.
I just upgraded to a macbook pro, but only because the Video card makes working on a portable so much faster.
So as long as rendering in HD in a hurry isn’t super important, I say go for it.