Remember if you buy the dual proc powermac, you will also have to buy a screen for it. Dell offers a 20.1 inch widescreen that is identical (same panel and everything) to the Apple screen for under $400 shipped, however, and it is quite a deal (trying to save up enough money for one myself, but Florida and Tiger are on the tops of my list now).
Remember with the dual proc powermac you also get the faster FSB, a dual layer 16x DVD drive, faster memory, and a better graphics card.
About the iMac's fans: In the 2nd gen (released a few weeks ago), they fixed the fan problem, from what I've heard at least.
If you decide to go with the PowerMac, wait until WWDC (early July). Rumor has it that dual core processors will make their way in to the line, and prices will be cut by as much as $500
