In terms of overall gaming performance, the AMD Opteron 6276 is massively better than the AMD Phenom II X4 820 when it comes to running the latest games. This also means it will be less likely to bottleneck more powerful GPUs, allowing them to achieve more of their gaming performance potential.

The Opteron 6276 was released over a year more recently than the Phenom II X4, and so the Opteron 6276 is likely to have better levels of support, and will be more optimized for running the latest games.

Both CPUs exhibit very powerful performance, so it probably isn't worth upgrading from one to the other, as both are capable of running even the most demanding games at the highest settings (assuming they are accompanied by equivalently powerful GPUs).

The Opteron 6276 has 12 more cores than the Phenom II X4. 16 cores is probably excessive if you mean to just run the latest games, as games are not yet able to harness this many cores. The cores in the Phenom II X4 is more than enough for gaming purposes. However, if you intend on running a server with the Opteron 6276, it would seem to be a decent choice.

More important for gaming than the number of cores and threads is the clock rate. Problematically, unless the two CPUs are from the same family, this can only serve as a general guide and nothing like an exact comparison, because the clock cycles per instruction (CPI) will vary so much.

The Opteron 6276 and Phenom II X4 are not from the same family of CPUs, so their clock speeds are by no means directly comparable. Bear in mind, then, that while the Phenom II X4 has a 0.5 GHz faster frequency, this is not always an indicator that it will be superior in performance, despite frequency being crucial when trying to avoid GPU bottlenecking. In this case, however, the difference is enough that it possibly indicates the superiority of the Opteron 6276.

Aside from the clock rate, the next-most important CPU features for PC game performance are L2 and L3 cache size. Faster than RAM, the more cache available, the more data that can be stored for lightning-fast retrieval. L1 Cache is not usually an issue anymore for gaming, with most high-end CPUs eking out about the same L1 performance, and L2 is more important than L3 - but L3 is still important if you want to reach the highest levels of performance. Bear in mind that although it is better to have a larger cache, the larger it is, the higher the latency, so a balance has to be struck.

The Phenom II X4 has a 1048 KB bigger L2 cache than the Opteron 6276, but on the other hand, it is the Opteron 6276 that has a 16380 MB bigger L3 cache than the Phenom II X4. The L3 size of the Phenom II X4 is probably low enough to greatly inhibit its gaming performance in comparison to the Opteron 6276.

The maximum Thermal Design Power is the power in Watts that the CPU will consume in the worst case scenario. The lithography is the semiconductor manufacturing technology being used to create the CPU - the smaller this is, the more transistors that can be fit into the CPU, and the closer the connections. For both the lithography and the TDP, it is the lower the better, because a lower number means a lower amount of power is necessary to run the CPU, and consequently a lower amount of heat is produced.

The Phenom II X4 has a 20 Watt lower Maximum TDP than the Opteron 6276. However, the Opteron 6276 was created with a 13 nm smaller manufacturing technology. Overall, by taking both into account, the Opteron 6276 is likely the CPU with the lower heat production and power requirements, but there really isn't much in it.