I'm with ketone. i don't think you'll receive a big enough payoff to justify only upgrading one of those. I'd save up and just get a whole new rig.

I'm going to guess you're not super proficient at troubleshooting because you're not sure which to upgrade. once you do upgrade you will for sure have a bottle neck. and if you don't know how to figure out where it is, you're going to struggle with your upgrade. So I highly advice against doing a single upgrade and just getting a new rig. (you can keep the SSD)

if you insist, i'd recommend doing the GPU first. It's easier all around to swap out and monitor. I suggest going up to a GTX480. the next jump after that one is the 680. don't bother with a 500 series, the jump in performance is fairly marginal compared to the 480.