This idea is similar to the setup I have now, with a PC hooked to a 24" Apple Cinema Display. I'm using a Sapphire Radeon 4890, which has Display Port, and I'm using the Circuit Assembly cable to adapt Mini DP to DP. Unfortunately, I was unable to use a XFX Radeon 5850. It just wouldn't work, and XFX and ATI were unable to help me. I've also heard from others using an Asus 5850 and XFX 5870 and neither of them were able to use the ACD on them either. Perhaps it would work on the iMac, but it doesn't look good right now for the 5800 series and the 24" Apple Cinema Display.
There are some powered converters but none of them say they'll support a resolution higher than 1920x1200.