You might like to get some more RAM for Dalaran and Northrend. I dont know much about the ATi Cards so someone else can comment on that.

If you use keyclone - you can use the PiP hotkey. I use F8 - F12 for my hotkeys. I bind F8 to my main window which is 1920 x 1200. I bind F9 - F12 to my other 4 regions which are split on my other screen which also runs at 1920x1200. Keyclone passes your PiP hotkeys thru to WoW now so I have 5 macros on each character that do this:

/clearfocus
/focus CHARACTER (the character that usually plays in that 'region')
/jamba setmaster CHARACTER

So that way whenever I flick between characters - say if one dies or I want to start looting on another one - my play experience is exactly the same as if I was playing my 'main'.

Since you have a Quad-Core I highly recommend letting all of your WoW clients use all 4 cores - Windows will happily manage the load by itself - you should notice a big FPS increase. Also if your card does AA/AF set it to 1x on all clients. Turn of v.sync. Turn of 'reduce input lag'.

If you have some kind of 'quality vs performance' slider in your graphics cards control panel set that to high performance as well.

Try cut down your mods as much as you can - obviously keep the ones you like - but you might want to sit down and think about if you really use or need that flashy fancy mod or if you hardly use it. Or do you need it on all 5 characters.