Here's a review that shows the GTX 570 and GTX 770 (among other GPUs) in a number of different benchmarks:

http://www.techpowerup.com/reviews/N...Force_GTX_770/

World of Warcraft specifically:

http://www.techpowerup.com/reviews/N...TX_770/24.html

The review covers the 2GB version, but the 4GB version is no different (except that it has more memory and costs a bit more because of that). It looks like you can expect a solid 30-40% of an increase in performance depending on what's going on in the scene (and how many clients you're running).

EDIT: I just guessed you were playing WoW, but I think you could be playing RIFT as well, which may not yield as much of a boost because that game is kind of a resource hog -- It's an upgrade none-the-less.