Dual processor Macs too hot??

I'm looking at buying a faster machine and I've read here that many of the dual processor Macs have a problem with overheating. Have any of you experienced that? It's said to be a problem particularly when more of the slots get filled with cards (as you might expect).

An Apple Store salesman concurred with all of that, but he said the problem has been solved in the recent designs. My problem is that these recent dual processor designs will not boot in OS9, according to the saleman, only OSX.

So, bottom line: I would like a fast machine that boots in OS9 but which doesn't overheat. I'll have all the slots filled, so....

Any advice from folks here, or experiences to share?

TIA