fwiw I was unhappy with dalaran performance a few days after Wrath. I was running 5x on a dualcore OC'd to 2.5ghz and a 8800GT w/ 4 gigs of RAM (3 gigs under XP). I moved first to this config:

PC1: c2duo 2.1ghz (unOC) w/ 8800GT and 3 gigs, running 3 clients with symlink'd WoW data\ folders
PC2: AMD dual core (4100? something like that, around 1-2 years old) with 2 gigs of RAM and 7950, running 2 clients with symlink'd WoW data\ folders

That ran ok for the most part, Dalaran still sucked but other stuff was a bit better, esp instances.

I upgraded a week ago to Vista64, added 4 gigs ($25AR from newegg!) to PC1, moved 2 gigs from PC1 to PC2 (so PC1 now has 6 gigs). I was a little hesitant that PC1 was going to suffer performance problems with Vista64, after all its a dual core PC thats nearly 2 years old now. But nope, no problems, Vista64 installed (new install, didnt upgrade) without a problem and Octopus/WoW are running fine. I don't really think Vista64 has improved anything a lot, but its nice having 64 bit and more memory to use up.

I still do get 6-15fps in Dalaran, but I don't have any load problems and all my guys have their hearth set to Dalaran. The nice thing is when I just want to play a single person I do so on PC1 (all 5 wow dirs exist on there too), only when I bring up all 5 does PC2 come into play.

Each of the machines has less than $600 in parts in it, probably even cheaper now, so its a fairly barebones cheap setup. The other nice thing is that in the future I can build a new cheap quadcore machine w/ 4870 or 260+, make PC1 into PC2 (and retire existing PC2) and I should be able to hit 30-60fps in Dalaran no problem.