If you're going to overclock, the Opteron will be better. They OC better, run cooler, and it has more cache.
If you're NOT going to overclock, the 4200+ will be better because it runs at a higher frequency. The extra cache on the Opteron is not enough to make up the difference in frequency in MOST games (some benefit from the extra cache).
Clock for clock and cache for cache, the processors perform nearly identical in gaming. They are the same CPU, except Opterons have been validated for use in servers (therefore more stable, cooler and rated for 24/7 running time).