I'd go with Xeons+ECC RAM. The Xeons are a bit more time-tested, and IIRC they have much faster performance than the Opterons at this point. The main benefit to the Opterons is the ability to address more than 4GB RAM, but if you're just doing a gaming server, that's not really necessary.
Make sure your Xeons have HyperThreading and you'll be a happy camper.
BTW, if you haven't already talked to RackmountsEtc on AIM about the price of that server, make sure you do. His prices are extremely low, considering how hgh-quality the servers are.