You'll see an increase in loading times for sure. Windows 7 memory management seems to be a lot better than XP and Vista both.

I initially started 5 boxing on a 2ghz dual core laptop with 4GB of RAM and windows XP (so not all RAM was usable). It was ok but lagged quite a bit.

When I moved to a quad core processor and SSD's I hit some limits because of the lower amount of vidoe memory (I had a radeon 4870, 512MB card). I had to upgrade to a 1GB 4890 to get rid of the bottleneck.

Basically there are a few places where you will hit bottlenecks. The sheer amount of data I/O when you load zones or textures (like in Dalaran for example) will tax your harddrive, an SSD will fix that, or at least make it as good as possible).

You should be fine on video too if you are only dual boxing. I've got my 4 "slaves" with "Low" settings and my "master" with settings on Ultra and it is fine. I get 60fps most times on the main and everything looks good.

If you do see any video issues you can always lower the screen resolution to help out.

Some other thing to consider when you start dual boxing is the Antialiasing in the video settings. Make sure you put that to 1 x (instead of 4, 8, 16 etc) because that will eat up all of your GPU...