The successor to the 9800 GTX didn't improve on it a great deal. It moved to the 55nm process, upped the core clock to 738MHz and the memory speed to 1836MHz, but kept the same 128 stream processors and 512MB of GDDR3 memory.
So the performance boost over the GTX was always going to be noticeable, but hardly stunning. In Crysis, the 9800 GTX+ averaged 37fps at high settings, with 66fps at very high in Call of Duty 4; the top playable rate in Call of Juarez was 33fps at medium, with 66fps in Far Cry 2 at high settings.
None of these scores is more than 4fps quicker than the bog-standard GTX. It makes slightly bigger gains at lower resolutions, but this is largely irrelevant, as you're far less likely to ever use them.
The original GTX is the 9800 GTX+'s only real price rival on the Nvidia side, and on ATI's side the closest to it is the $273 HD 4850 - which is $75 cheaper.
You may think this puts the GTX+ in a good spot - surely, with no price rival it must also occupy a little performance band of its own, right? Wrong. That $273 HD 4850 stays just a few frames per second behind the GTX+ in the tests that matter - certainly well within the price gap that exists between the two.
When you add in the fact that the HD 4850 is a much smaller card, with a less noisy fan and only a single six-pin power input to the GTX's two, the GTX+ begins to lose its appeal. True, it may be a tiny bit quicker in some of our gaming tests, but as an overall package for gaming and media we'd choose the cheaper HD 4850 every time.