You would have to outline your usage.
What games if any do you play?
Video Editing?
and most importantly.. What would you feel comfortable spending? Its easy to fiddle with part specs to adjust price.
You could get away with a really nice gaming system for a little over $1700ish with OS, 20" WS etc.
It can be done for $1500 but obviously skimping a little on cpu speed, possibly gfx speed also. All depends on your needs.