Not sure what charts you looked at, but on the charts section here the 965 beats the Q6600 in every category 965 VS Q6600
It beats it in games by a fair bit, boosting performance in GTA IV from 50 to 62 is the most significant, but these days your gaming wont really be limited by a decently quick quad core, once the CPU performance is at or above 60FPS it doesnt matter as the monitor's refresh rate becomes the limit then.
You will take a performance hit from the downgrade to 2GB of ram vs 4GB of ram but the speed of the ram only affects performance by a few percentage points at most. Your new ram would probably perform better but overall system performance will drop a bit due to it having to access the HDD more often because it cant store everything it needs to on the ram.
If you can get a Q6600 up to the ghz of the Phenom II it will perform surprisingly closer.
Not saying the Q6600 is any match, just that deneb isn't too far ahead clock for clock to kentsfield, especially in games. Don't get me wrong, Deneb is superior in every way, and has much higher OC capabililties.
The PII 965 is the better compared directly with the Q6600. Had you not obtained the PII 965 & motherboard for $260 I'd have suggested you take a look at the i5-750 instead.
But it sounds like you made out decently. Keep the 4GB of RAM too. Overall it'll perform better than 2GB. Keep in mind however, that with Vista 32bit, you won't see all 4GB of your RAM. It's not capable of using that much, especially if you have a GPU installed. 32 bit Windows can only use so much RAM + VRAM.
You might consider 64bit Windows next time you reinstall or upgrade.