Only noticeable in current dual core utilized games like Quake 4, the ones Marc mentioned, RB6 Vegas, and a few others have decent utilization of 2 cores. I posted a similar thread and the general consensus from people who did basically the same upgrade was NO difference.
Yet in spite of that, I still did the same upgrade.

UPS is taking their sweet ass time cause they wouldn't want to be early or anything like that, but I can update on Monday when it arrives. I'll be going from a Opteron 146 @ 2.4 GHz to a Opteron 165 which should have no problem hitting 2.4 GHz, and I will likely be mobo limited at ~300x9=2.7 GHz before the chip limits me (latest ones from Newegg reviews show good overclocks).
So at the 1680x1050 I game at, it is unlikely to see any performance boost. But it is possible I'll end up with a slightly higher clock speed from the 165, so I'm not loosing anything. The 165 was $94 shipped minus $35-40 (est.) for selling my 146, so a cheap upgrade still. I fully am aware to not expect really anything in the majority of games I play, but this upgrade is so that with a new video card, I will be able to play Crysis and other games. Hell, Shadowrun requires a dual core processor already.
Another reason that I upgraded my 939 system is that it is unlikely that 939 chips like the Opteron 165 or lower X2s to drop that much more in price (unless you consider used). There is still a demand for them. Since I won't be able to afford a new C2D or other system AND a new video card in the next 6+ months, I figured a cheap dual core to take advantage of upcoming games (and not lose any performance in non-dual core games anyway) along with a new video card (8800GTS will drop in price in the upcoming months, while a dual core 939 like the 165 is unlikely to drop much more) will hold me over for about a year.