The flashed X800XT (FireGL) is a good card.
Running at Apple OEM X800XT clocks, it is slower than with the FireGL clock. The card, though, running faster (at FireGL clock speeds and faster) heats things up fast. I have one OC'd to 550 MHz, but also have a Arctic Cooling ATI Silencer on it.
It's a bit of a struggle to cool the FireGL with it's stock cooler, even at OEM ROM settings.
The flashed X800 is a single slot card, good for PCI card placement, has a dual link DVI port and provides very strong OpenGL support for OS X.
Weak coolers follow previous Radeon cards.....
I'd get the Geforce 6800 Ultra: