The Mac Pro simply isn't a gaming machine for 2 reasons: dual Xeons and FB-DIMMs.
At this time, as had been said, very few games make much use of multi-threading/benefit from it. In some cases, performance diminishes when multiple CPUs are added to the system.
And FB-DIMMs. New serialized RAM basically. It promises higher throughput, but right now, well, it sucks, especially for games. It's twice as latent and twice as expensive. Anandtech did a nice technical article about it.
So, I, being a gamer and a Mac lover, am in a bind. I can either stay with my slow G4 mini plus Pentium-D game box, keep the mini as Mac but upgrade the PC, get a new Intel mini and keep the PC, or go hog wild and buy a Mac Pro to replace both. The 24" iMac is also a consideration, but not quite a gaming box (slab?) either since even the 7600GT option is only mid-range and there's no aftermarket upgrade available.
I'm holding out a sliver of hope (in vain, I'm sure, knowing Teh Steve) that Apple will come out with a mid-range Conroe Core 2 Duo based Mac that uses regular DDR2 RAM and at least 1 PCI-E 16X slot.
The other day, I also had an interesting thought regarding the possible next generation of consumer computers. The 24" iMac was the first ever to use the MXM module, but it's not user-accessible and not advertised as upgradeable. What if Apple, with the next revisions, put MXM modules (and access doors!) across the Mac Mini, iMac, Mac Book, and Mac Book Pro lines? And offered a number of build-to-order options, as well as aftermarket upgrades?
Gamers would be happy, and the current stagnant 3rd party Mac upgrade market would be revived! The original iMac could be upgraded with a 3dfx Voodoo card, and there's still processor upgrades for the old G3/4 Macs that have socketed CPUs, so why not?