1- The Opteron series is a server processor, not for gaming.
2- Only support DDR2 ('till I know)
3- With the price of the Opteron processor, you can get a i7-920, i7-860 or even, the i7-975
Why would the OP consider buying obsolete Intel CPUs? Sandy Bridge was released in Jan 2011.
Opteron is a server processor as has been said. It is not optimized for gaming and even if it was the low clockspeed ( 2.8Ghz ) would cripple it. Most games can only use 2 to 4 cores and benefit from a few faster cores not more slower ones. If you want to spend that much on a CPU get the i7 3970X. That is the absolute best gaming CPU you can buy although it's overkill 99% of the time. Even the i5 3570K is faster than any CPU AMD makes for gaming.
If you need an i7 get the 2600K or 3770K. If you need more than 4 cores get an LGA 2011 setup using an i7 3930K, 3960X or 3970X. The i7 965 is an older architecture.
As I said above though no games need more than the i5. The Hyperthreading in the i7 can actually hurt performance in some games. The i7 series is more for video rendering/encoding and running VMware and massive multitasking. The i5 is the best choice for just gaming.