I guess it will do just fine if you install Vista on it.
The problem with Macs and gaming is not as much the hardware as the poor quality of the games that are ported to Mac OS. The Mac Pro is also quite pricey for a gaming rig, but since you said your last Mac was from 1990, I take you are not kid anymore and earn your own money. So I won't point any fingers at you there either. Availability of graphics cards that are Mac OS compatible might be a problem for some people, but you can always have stock card for Mac OS and a GeForce 8800 GTX for Windows gaming. Someone might be able to explain this topic better than me.
The issue I think might be the biggest problem is that the Mac Pro's FB RAM has a higher latency than ordinary DDR 2 RAM, but again I only think this is an issue for the very serious gamer.
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