A quad core processor will be fine, but I'd recommend an i7 920 for 5-boxing if you're buying a system now.
At least 6GB or ram recommended, more if you'll be doing other things while boxing.

I went with an Asus P6T motherboard, but there are a lot of good choices out there.

You'll want a 64-bit operating system, so you can use more then 4gb of ram in the system.

A cheaper SSD (solid state drive), purely for the gaming folder is recommended.
You won't need much space, and as long as you're not putting your operating system on it, a low end SSD will be fine.