9800GTX to GTX260 core 216 performance increase?

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.

In UT3 OC your cpu might do more then getting a GTX 260 you'll see an improvement there is no doubt but it would be minimal and probably not worth it what's your resolution?

He'll get more value from the 9800GTX if he overclocked, too.

3dmark06 has not been a viable benchmark for years (if ever..).

While not everyone overclocks the CPU, to play at such a low resolution as 1440*900 and not OC you are really hampering things. Though the CPU is fine and dandy, just not running as fast as it could .

For that resolution I would not advise you to upgrade at all frankly. Perhaps OC a bit, but certainly not replace a 9800GTX, it is plenty.

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.