I don't want something that will be technically obsolete in six months. I bought my current Dell 8250 on 12/29/2002. If it wasn't for making a mistake in getting a Rambus-based motherboard, I'd upgrade it. But then I'd still have only Windows.
Besides, 2gb of Rambus memory (4 x PC1066 512mb) costs more than the 16gb of memory for the Mac.