When I had the problem in my system I was running it on as 72-00 RPM 180ish GB hard drive that was about 75% full (the idea is any hd over 50% full shows a performance drop). I had 2 SSD 32 GB hard drives, one of which i used for my normal single player wow (different addons..etc) and the other was running as a backup OS.

I reformated it, droped a copy of my multiboxing WoW on it and things got a lot better. Id say I received an increase of about 10fps from dropping it on my SSD in the larger battles. I didn't think it would make a difference, but looks like a lot more loading was going on in AV then expected during the larger battles.

I don't think i was maxing my RAM...i only 4 box and dont creep up too far past 5GB of RAM usage anytime im looking. During these trouble times I was running the nvidia system monitor to keep tabs on everything.

Windows Vista could of very well been doing something anyways with the drive during the larger AV battles.