After reading everything here, so many good advices makes it hard to give my own. I did a ton of testing on my rig, although inferior (dual core), I have a few things to add.
In the beta, then in 4.0, SLI and ultra water would drop frames about by about half. There was something else with ultra shadows and the water, too, with SLI. I think CF was affected as well, not sure though. The new area, Vashjir, is an all water area near Stormwind and hit my system hard even with laid back settings at High, had to drop a few to Good to stay above 45fps.
I know my dual core holds back my performance a lot because I can see max cpu in a few key circumstances. In raids I get 60fps, until we engage the boss, then I drop to ~25fps even overclocked to 3.85ghz. Same drop in Dalaran at peak hours. I get it again with max distance + max shadows outdoors with a lot of trees. As others said, one notch under ultra shadows is key ("Good" shadows). "Good" view distance also seems to help [me].
Last thing I noticed. GPU ram was maxed out in some cases (1gb) and if you went over it probably would drop your fps. It does exactly that on my laptop, but to the extreme. You might be over the 2gb vram using Eyefinity, especially when other's suggest dropping AA. That's exactly what I have to do on my laptop.
I monitor my system using dual monitors and have CPU, GPU, and NET usages/temps on desktop. Even with my inferior system, I never lag when alt tabbing in windows 7. Now in Vista, alt tabbing would take a while.
I use the following gadgets and they work great, been using them for 2 months now (first one requires CoreTemp for cpu temps)
http://addgadget.com/all_cpu_meter/
http://addgadget.com/network_meter/
http://blog.orbmu2k.de/sidebar-gadge...sidebar-gadget
http://blog.orbmu2k.de/sidebar-gadge...sidebar-gadget
Hopefully this helps some people troubleshooting their system usage.
Ind
Edit: Oh also since I'm new, hello. And if it helps for rep's sake, I am
Indpendent (gnome mage) of US-Bronzebeard.