If you want to crank up in-game settings and resolution, then it might be a good move.
But as of right now, with your current GPU, you're technically just fine.
Personally, I went from a Radeon 9800 to my 7800 GTX, but I can honestly say that I don't feel I should have done that. My 9800 Pro served me well enough. Of course, now I can play at higher resolutions, and pretty much any, and all in-game settings at their highest possible ... but it's not like I would have died playing BF 2 at medium settings at 1280 x 1024, instead of high settings at the same resolution with some AA and AF. Differences in visual quality IS indeed present, it IS noticeable ... but it's nothing to be blown away by.
To "skip" a few generations is, as I personally believe, the best thing to do, before upgrading a GPU.
In you situation I would certainly wait either for nVidia's next generation (G80) or ATi's (R580).