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Yes, you can buy the kit and install the card yourself. It is about $275 from the online store.
 
You could also buy the base model, and add in two 3870's. Cheaper, and they do better at Core Image. The only niggle might be the cooling fan. Some users have complained that they're loud. 3rd party passive coolers are available though, and IIRC, added in, would still be less $$$ than the 8800 Apple sells.

If you're willing, another possibility exists. Buy 2 PC versions of the 8800 that have a 128KB ROM, and flash it.

There's threads on both the cooler and about flashing the 8800.
 
Remember Nvidia SLI is not supported on the Mac Pro at all! (not even in bootcamp windows)


I would get the 3870's, the fan issue is because of defective cards, mine is quiet. (crossfire will work in bootcamp)
 
What kind of performance could you expect out of 2x 3870's in Windows? I just got my 8-core 2.8 (only 2 gigs ram though for now), but just have the stock video. I've got games ranging from old school (Jedi Knight 2) to Age of Empires 3, FEAR, Far Cry, etc. I am not necessarily worried about running MAX settings at an ultra high resolution, but I my stock card will likely choke bigtime on some of these games.
 
What kind of performance could you expect out of 2x 3870's in Windows? I just got my 8-core 2.8 (only 2 gigs ram though for now), but just have the stock video. I've got games ranging from old school (Jedi Knight 2) to Age of Empires 3, FEAR, Far Cry, etc. I am not necessarily worried about running MAX settings at an ultra high resolution, but I my stock card will likely choke bigtime on some of these games.
You're not going to need 3870s in Crossfire for that...

I'm going to strongly suggest waiting for the HD48xx. The leap for the price over the HD38xx puts the older architecture to shame.
 
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