Be careful when shopping for FX 5900 cards for Shuttle systems. While they don't all have massive blowers sitting on top, some do have larger then normal heatsinks still that will hang out the side of the Shuttle. I'd check some place like the SFF Tech.com boards for compatibility.

Memory wise, 256 might be a bit much. All hints from id were pointing to probably 128 needed to run Doom 3 with the highest quality textures. Games will probably start going beyond 128 sometime in late 2004.

ATI, They do make their own cards as well, but I hear in Europe, the "powered by ATI cards" are more common. In most cases, they are the same board design, but with a different company actually doing the manufacturing work. Overall, I have heard good things about the Hercules boards.