Here is a GREAT comparison of Opterons, dual-core Opterons, Athlon64s and Athlon64 X2 processors in the budget range, with Intel processors thrown in. You can see that Opterons have no issue keeping up with Athlon64s in gaming.
http://www.anandtech.com/cpuchipsets/showdoc.aspx?i=2736&p=9
The Opteron 144 is a single core, and 165 is a dual-core CPU.
All dual-core Opterons come with 1MB cache, therefore there is no equivalent Opteron to the Athlon64 X2 3800/4200/4600+. The Opteron 165 runs at 1.8GHz, and the Opteron 170 runs at 2.0GHz. The performance of the Athlon64 X2 3800+ will lie somewhere between the two on average.
The Opteron 165 has more overclocking potential over stock than the 3800+. If you are not overclocking, the performance of the two are roughly equivalent, as the Opteron 165 has double the cache, but the X2 3800+ is 200MHz faster.