Ask HN: Looking for a (literally) cool laptop - Any advice?

Dear HNers,

I recently bought a dirt-cheap emachine laptop, and unsurprisingly got what I paid for, specifically regarding overheating: it burns me. While I can't afford right now a more expensive computer, I'd like to read your advice on cool alternatives. (+)

I'm planning to run Ubuntu on that machine, but your impressions on MacBooks are welcome too.

Thank you!

(+) I know it's clear by context and title, but please bear with a non-native speaker: I feel compelled to emphasize that by cool I mean the literal meaning of cool. (Low-heat would have been simpler, huh?)