Middle of the road? dang, what's the upper end?
i7 920 is pretty powerful as it is, unless you move up to a higher clocked i7 or Xeon, or multiple CPU's, not much better than that.
GTX275 is only a step down from the 285, and the 285 is the most powerful single GPU card out right now. only option better, until ATI's 5xxx series comes out this month, is to step up to a GTX 295 really but thats dual GPU's.
12GB's should be more than enough too.
only running 3GB atm with my 920.
but you should notice a fairly big increase over the X2 5000 easily.