Just my 2cp from someone who's just built himself a new rig and recently started 5-boxing.

I went with:

mobo : Asus 6pt
CPU : Intel i7 920
CPU Cooler : Corsair H50
Gfx : ATI Radeon 5850
RAM : 6gig DDR3. (some recommend 12, but 6 gig overclocks better and is more than enough)
Case : Antec 902 (lots of room, quality build and enough fans to create your own wind tunnel!)
PSU : Corsair 650

I've got Windows 7 premium and 1 copy of wow on a 64gb SSD drive. (look for 200/150+ read/write speeds) I use ISBoxer to sort out my character settings for me, which so far i've had no issues with at all!

Keyboard / mouse / headset / x-keys etc etc are generally personal choice. I spent around £100 in total on them though.

All in all it came to £1400 for me. I've overclocked the CPU easily to 3.8, and the GFX Card to about a 15% clock in total.

With the above, running on res of 1920x1080 through my 37" TV, and 1280x1024 on a 19" for my slaves; i get a steady 60fps on my main, and 30fps on slaves. Even in Dalaran. That's with max settings on my master, and medium settings on the slaves.

Hope that helps a little.