Things to keep in mind:
My money is very tight, therefore bang for the buck is the be all/end all. I barely game at all. I've been using a 9500GT on my current build for the last 2 years and it's been fine for my needs. I plan to use something of similar scale in this build. If I do game, it's either WoW or Civ or strategy games of the like. The thing I do the most, is I have a ton of apps open at once. I also love to overclock my processors -- again, getting the most bang for my buck.
With the price/performance in mind, what combo would you choose and why? Also, how much of an upgrade will this combo be from a 550 black edition OC'd to 4GHz paired with a Gigabyte GA-MA790XT-UD4P? I'd love to hear any and all comments/suggestions.