I don't use vantage so I don't know about the score, or if this thread is too dead for you to reply to buuut... the Q6600 @3.0ghz shouldn't bottleneck the GTX 260 and make sure you are searching for CPUs with the same frequency, a lot of people have some wickedly clocked Q6600s out there.
Also I know 3dmark06 doesn't show if the person is using SLI/crossfire so watch for that on vantage (which I'm guessing they would fix but since I don't know *shrug)
Also, if you are using windows XP you should stick with your 2 faster A-data sticks if they are 2GB each. Try taking out the OCZ stick and see if you get an increase or decrease in performance, even though I'm sure you have tried this. (and I'm not saying this could dramatically change your vantage score, but other performance on your PC). Also a FYI a 32bit OS will not use more than 3.2GBish of ram so watch out!