OK... Quick follow up. On Aqua-Mac, the guy has an X850 Pro flashed and running in an MDD, of all things. Hurray! At first I freaked because it's cooling is so big it hangs over two of the four RAM slots, but then I realized he had fitted the X850 XT's cooling fan to it in order to safely overclock the X850. I have no desire to do that, so the plain X850 Pro (which is noted as single slot) should work fine.

I can borrow an old ATI PCI video card in order to hook up my monitor when I do my flashing. I am so excited! Woo!
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DP 1.08GHz PMG4 MDD, 1GB PC2700 DDR, 200GB ATA (16MB, 7200RPM), Radeon 9800 Pro (256MB)