Here's my dualcore box:

DualCore @ 3.2
4gb ram
8800GTS 512MB
Main at "ultra", slaves at lowest / capped at 15

1-60 (old world): 40fps+ on all screens (uncapped)
60-70: 30 on main, 40+ on slaves (uncapped)
70-80: 25 on main, 30+ on slaves (uncapped)
Dalaran(med. pop PVP server): <10fps on main, 10-15 on slaves (capped)
Ogrimmar during peak time: <20fps on main, 10-15 on slaves

This is with quest helper running on all boxes too, which is a bit of a memory hog and xperl / bagnon, etc. on all windows.

In my experience, running instances and doing regular zones (even wotlk zones) that box is fine as long as your slaves are set to lowest and add-ons are minimized. Dalaran looks like it will require at least an i7 with the proper affinities set, 64 bit OS, 6GB+ ram, and a fat pipe so if I were you i'd just park your guys somewhere close (like in the ledge on Krassus landing) and run them through town one at a time.