That means the 3 gpu rig costs 20% more.
You can get 2x (58xx or 6xxx using a total of 300 watt PCIe power connector) and 2x5770 (150 watts of PCIe) cards into one case. I know this because I'm doing it. That keeps the cost down significantly because you can run a 600-700 watt PSU depending on CPU. And when you get over the 3 box hump you can't realistically (unless you feel like spreading parts around on a large table or baking rack and filling the room with oscillating fans) put them in one location without looking like a degenerate that doesn't care about consistency and stability.
4 cards per box is the best use of space when you're dealing with 3+ boxes. There's also the power distribution, networking cables, environment cooling and heat exhaust systems, and administrative work to consider. Having all of your gear in an orderly arrangement keeps you sane. I know a lot of people here love open cases but coming from a datacenter work environment it just makes me think you're playing with toys when you could be approaching it in a serious manner.
And they haven't cared about the "non-transferrable" part of XP licenses since 2007 when they stopped selling them.
I think it's WAY easier to set up than Windows, but to each his own.