I'm planning to get the new 24" iMac but am wavering whether to order it with the stock 7300GT/128mb or upgrade the video to the 7600GT/256mb. Naturally I'd prefer the better card (for xplane-8 (http://www.x-plane.com)) but I am concerned that the faster card may have an onboard cooling fan that could increase noise levels. Does anyone know whether the version of the 7600GT used by Apple in the iMac24 is actively or passively cooled?

Since the iMac uses a laptop graphics GPU, it's not on a card as you know it (I'ts a really small card - daughtercard size), it's more of a module. So no there is no fan on either of the graphic chip modules. There is no space for such a thing. The iMac cooling system is probably responsible for keeping the graphics GPU cool. Maybe there's a heatpipe or heatsink specifically for the module.

Here is a link of what it would look similar to: http://www.hothardware.com/articleimages/Item524/small_module1.jpg

more:
http://www.hothardware.com/viewarticle.aspx?articleid=524

Yea don't expect full performance (my iMac's ATI Mobility x1600 is double the performance of my old desktop ATI 8500 but half the performacne of a desktop ATI x1600 but the performance is still really good for current gen games (Quake 4, Need For Speed Most Wanted...), I can play at 1280x800 at very high quality no problem.

I've seen posting that come from people with access to the Apple service manuals. Apparently there is a heat pipe connected to the GPU.

So, I'd say it's passively cooled, but I expect there is a fan that would kick in if required. It may be the system fan. I think Apple may also be able to throttle the GPU's speed if required to keep the thermal window correct.