* Radeon X1900 XT ($399)
* Radeon HD 2600 XT ($149)
'Cause, unless there's some reason that putting the 2600 into arcturus next to the 1900 it's already got is really stupid, I don't see any reason to spend the extra $250.
edit: It's also interesting to note that I've seen a lot of people complaining about the 1900s they got in their macs, and macenstein notes that apple is apparently swapping out defective ones. I haven't had any problems with mine yet, happily. (All the problems I've been having are around the bluetooth and mouse/keyboard weirdness. Both of which I should probably bug apple about soon.)