StarQ, it will be the best thing you ever did. Definitely put the 8800GT card in.
Honestly, I can't believe I have to defend a Mac for gaming to a bunch of Mac Enthusiasts....I'm used to such defenses against my PC friends, but here I am surprised.
I've been on Apple since AppleIIe, and I also build custom gaming rigs. I've built quad cores with SLI and Raid Arrays...and this machine outperforms them all.
The Mac Pro 8 Core is the best computer I have ever owned and it ROCKS FOR GAMING. I say this 8 months after I've purchased it....usually after 8 months I'm looking for the next best thing.
I will never build myself a custom gaming PC again.
Listen, I am an pro audio guy. I am also a semi pro gamer. This machine handles ALL my needs. From Video editing in After Effects, to Sound recording in Cubase with a Motu-828 to Flash Development. At the end of the day, I boot windows and rock out on some CoD: World at war.
I used to play competitively in CAL and TWL, so I know about changing config files and removing the FPS cap. I had CoD4 getting 350 FPS on this machine on a custom config I wrote.
The only downsides are lack of sound card, and lack of inexpensive installation of Raid drives...but honestly I do not miss them. I am actually thinking of putting in an Audigy-ZS from an older computer knowing it will only work in windows for 7.1.
Don't listen to these guys who say this is for professionals only. Buy one for ALL your needs, you'll be happy you did.