*If* you're going to buy a new cpu and are planning an spending more than $100 on it, the Q6600 is by far the best deal for around $260. However, I'm running 5 instances on an overclocked E6600 and am satisfied with the playability. My current complaint has to deal with dual-view from an 8800GTX over two different sized monitors, but once my 2nd 24" arrives, I'm sure I'll have better frames. What size monitor(s) are you running? Are you using dual-view or horizontal span? If you're using dual-view, you can expect far less than stellar performance from your videocard, regardless of model. Your 256mb 7900GT should be able to run multiple instances of WoW across 2 20" monitors in horizontal span without too much trouble. Since you said you're not happy with your single instance of WoW performance, it's either your CPU or videocard (or memory if you've got a ton of other apps in the background). You can't go wrong with quad-core or the 8800gt, but knowing what resolutions you're trying to run the game at would help..

As for memory, I'm running 4GB on WinXP 32bit (~3.2GB available), and 60% of it is in use with 5 instances of WoW up, so since DDR2 memory is so cheap, go ahead and get the full 4GB (~3.2GB in 32bit).