Your PC - to be frank - is ill equipped for Multiboxing.

For starters

New Motherboard.
New CPU, Quad or Dual core.
RAM 2GB+
GPU Nvidia 8600GT 512MB(+)

Have you tried disabling the Nagle algorithm? That should knock around 40-50ms off. Next time you are ingame, fire up a command prompt and do a tracert to the WoW server you are playing on. That should tell you *WHERE* your connection is bottlenecking.