Yes it will improve your framerates but only slightly. You'll need to overclock your CPU to get the most out of the new videocard. It's like getting a porsche and driving on an empty freeway but there's a mechanical device under the gas pedal that only lets you step on it halfway. That device is your CPU.
At 3.0 Ghz, my e8400 and XFX GTX 260 gets a 14K score on 3dMark06. At 4.0 Ghz, it gets over 18K.
Exactly my point. You'll get more value from the GTX 260 with an overclocked FSB. Why put in a high performance card if the rest of the system won't keep up?
I don't think he'll see a major difference in most games, although UT3 is somewhat CPU bound because it's not particularly taxing on the GPU. When you're talking about increasing your framerate with a CPU overclock from 100FPS to 110FPS it's not much of an issue anyway. 3.2GHz is a mild OC that should give the system that extra bit of grunt and should be easy to reach so it's worth the miniscule effort.
you have to wonder as well as to how much that score increase affects real world game performance.
In Supreme Commander it means alot. In most games you'd be lucky to see a difference.
To sum things up, it looks like the majority of the posts are saying no you won't see a significant gain. Your Quad Core is sufficient now just needs a slight overclock to push the 9800 a little harder if you feel the need to do so.