I have near the same setup and I only lag in Shatt.
Windows 64-bit Vista
8800 GTS 512 Mb vid card
4Gb RAM
quad-core 2.4Ghz proc
I use 1 standard WoW folder, each copy getting maxfpsbk 30 and maxfps 45, cpu affinity NONE.
1 19" monitor 2x2 grid setup for my toons.
Maybe 5 seconds to get out of a bg.

Normal usage is around 30% cpu, 50-60% ram, Shatt is around 70% ram.