What do you want to use it for? This is my area of expertise. Are you capable of/willing to overclock?

But I don't think I'd agree with anything Bob listed.. maybe one thing..

Here's my opinion. Computers change so fast.. that it's not worth spending your entire load on something now. Maybe limit yourself to around $1000 and put the rest of the money towards an upgrade in a year or so. Let me know if you just want to go all out, or want to be rational and budget it, though..