If I had $5k spare, this is totally what I'd get, possibly the coolest PC ever (pun intended)
http://www.maximumpc.com/article/features/hardcorepc_reactor
That looks pretty win.
Of course, it could be a real pain if you want to, say, swap in a new video card, but depending on how much extra processing power you can get out of the thing, it might just be worth it.