People are making it sound like the Mac Pro is a mediocre gaming machine. That is complete crap. My Mac Pro 2.66 with an X1900 XT runs BF2, Far Cry, FEAR, all those intense games absolutely spectacularly.
When people criticize the Mac Pro as a gaming machine, it's not because it's somehow bad at it, but because you could get the same kind of performance for much less with a non-workstation gaming PC.
Also, people exaggerate the performance drop between the Xeon using FB-DIMM and the Core 2 Duo using normal DDR2 RAM. It's not like comparing an Apple II to a Cray at all.
Here's why: games these days are very much GPU-bound, meaning that there is increasing stress on the GPU and relatively less on the GPU. What this means is that any disadvantage the CPU and RAM may have to the non-Xeon counterparts is anulled by the fact that most new games coming out are bottlenecked by the GPU.
The bottom line is that all current games, such as Battlefield 2 and Half-Life 2, run amazingly on the Mac Pro. Don't let anybody confuse you on this point. It really comes down to how much money you want to waste on power that won't be reflected in games as much as in professional creative/scientific work.
I never said and I don't think anyone did say that Mac Pro was a mediocre gaming machine... its just that for the money from a solely gaming perspective its too pricey, thats all.
Xeon is just a Conroe, in fact its better as it has a 1333mhz FSB and multi processor support so with 4 cores so no worries about the processor. As far as videocards go... you can use anything with Mac Pro, you can only not use SLI/Crossfire but thats hardly a disadvantage, there ARE single card multi GPU solutions and in any case, a single top of the line card will play any game at 2560x1600.
Then come FB-DIMMs, an obvious disadvantage is price, costing almost double of what regular DDR2 costs, Anand benchmarks shows up to a 30fps penalty... sounds bad but its necessary to consider the fact that it went 30 fps down from an already staggering 130fps AND they were running at 1024x768, resolutions that are CPU/RAM bound, at higher resolutions and settings that difference should close considerably. In any case, I don't think anybody is going to whine for losing a few FPS off such high performance.
So yes indeed, Mac Pro is a great gaming machine and an awesome deal for what you get but from a STRICTLY gaming perspective point of view... there are far better deals.