My setup is like this

C: = slower drive for Windows. Having a 10k C: will just make you boot faster, lol.

E: = 2 Raid0 10k 74GB drives
- World of Warcraft
- WoW0
- WoW1 - /data, /cache, /interface folders are symlinked to e:\World of Warcraft
- WoW2 - /data, /cache, /interface folders are symlinked to e:\World of Warcraft
- WoW3 - /data, /cache, /interface folders are symlinked to e:\World of Warcraft
- WoW4 - /data, /cache, /interface folders are symlinked to e:\World of Warcraft

Symlink is basically a fancy shortcut. I symlink my Interface folder to make updating addons easier :/

I'm running Vista Ultimate 64 bit and I also use an ATI 3870, just one, and I have 0 lag now.