To give you a feel for the differences in Dalaran. My alt (4-box) system was:
  • Core2Duo OC to 3.1GHz
  • 4GB RAM (normal 1066 stuff, nothing spectacular)
  • Your basic SATA-300 non-raid, non-high-performance model :P
I was having issues in Dalaran. Alts would lag behind so bad I'd have to run 20 yards, stop for a second, run 20 yards, stop, repeat, etc. Real pain in the butt.

So, I swapped the Core2Duo out for a Q6600 2.4GHz OC to 3.1GHz and I don't have to wait for the alts any more. Granted, when I first get there, all my systems chug a bit from a bajillion alliance scrubs hopping around in circles in their raid purplez. Aside from that, no difference in graphics, motherboard or HDD. The CPU there really does help a ton.