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.