Let me start out by saying that I am by no means an expert at multiboxing. However, I'd like to think that I'm more than competent in most things involving a computer.

So far, since I've installed W7 RC1, I haven't had any issues. Didn't need to stop any services or settings for anything other than WoW. Here's my hardware, so you can compare, you may have to do something more, but here's my experience.

Intel Core2Duo 2.66GHz (OC'd to 3.0)
3GB DDR2 6400 in dual channel
2x nVidia 8600GTS 512MB in SLi

I keep a copy of my working WoW directory just in case anything goes haywire, so I'm running 2 instances of the game on one rig (but from different HDDs), usually on one screen since I can see my alt behind me. I killed all the settings for my slave, all low settings, but I have it on 1280x768 res still, 800x600 kills my face - and my main is full settings1600x900 resolution w/ a good many addons. Keyclone works for me no problem. I run at well 50fps on my main and more than double that on my slave.

Like Lax said, make sure your VGA drivers are up to date, that's gonna be probably the most important thing. I've had no issues w/ the drivers working in the RC, but did in beta, so it's definitely gotten better.