I had to move from 4gb windows xp to 8gb Vista64 to get dalaran playable, my current specs are:

Striker Extreme Mobo
Q6700 Processor
8GB Ram
GTX 280 SSC Graphics (overclocked by about 20% with evg's precision tool)
Vista 64bit (only thing ive done with it is turn aero off)
1 raptor drive per wow client (7 harddrives total in my machine)
Tons of well placed fans

I run pretty decent in Dalaran now, usually 8 to 15 fps and my current bottleneck seems to be my CPU, it runs at 100% on all cores when I am in Dalaran. I'm thinking about OC'ing it to see if it improves performance.