The Opteron single cores, 144, 146, 148, 150 all have 1MB of L2 Cache, while only the 3700+ and higher have 1MB of cache. The Opteron single cores have been seen to OC really well, even with lower stock vcores, some have gotten up to 3.3ghz or so.
The Opteron dual cores, 165, 170, 175, 180 all have 2x1MB of L2 Cache, while the X2s all have 2x512KB of L2 Cache, except for the X2 4400+ and 2 4800+. The Opteron dual cores have also been seen to OC better then the X2 counterparts, and at a lower vcore of course.
Opterons can be considered good for gaming, especially since if you get the 165, which is a dual core. I'd say it's definitely worth the extra cash over a 3700+ to go for the Opteron 165, you get the benefits of a dual core and good OCability at lower voltages.