This spring I upgraded my E8500+gtx260. In two steps.
Step 1: replace gtx260 with a gtx680.
My framerates kinda doubled. Maybe a bit less. When I looked at benchmarks, I could see that the gtx680 was supposed to be 3x more powerful than my old gtsx260.
There was a side-effect. Which could have been expected, but I hadn't realized it. With my new gtx680 I could enable a bunch of extra eyecandy, and hardly lose any framerates. More AA. SSAO. High-res texture packs in Skyrim. Etc. My framerates would hardly go down. But if I disabled eyecandy (low settings) I would hardly gain more framerates too.
Step 2: replace E8500 with i5-3570K.
Now my framerates would go up another 50% or so.
Example: Skyrim, top of the stairs in Whiterun:
18 fps with E8500/gtx260
32 fps with E8500/gtx680 (and more eyecandy)
45 fps with 3570k/gtx680
I gained another 5 fps or so with new drivers.
And another 5-8 fps or so when I overclocked my CPU from 3.4GHz to 4.0GHz.
And then a few more when going from a 1920x1200 to 1920x1080 monitor.
I am now at 60 fps. Those stairs are not a good benchmark anymore.
Skyrim is known to be CPU intensive. So are MMOs. So are RTSs. In these games a new videocard will allow you to configure more eyecandy, but maybe not double your framerates. In other games (which depend on GPU more) you might see a bigger increase in fps.
My suggestion: upgrade your GPU first. Enjoy the better eyecandy and the slightly higher framerates. Then later when you have more money, buy a new CPU/mobo/RAM.