Elitebastards take a look at the difference in performance between the two different iterations of GeForce GTX 260, namely the Core 192 model versus the later Core 216 part. Here's an excerpt:

I'm pretty sure that we could win an award for the most blatantly obvious article conclusion ever if we simply noted here that "the GeForce GTX 260 Core 216 is faster than its predecessor with just 192 Stream Processors", although in fairness that conclusion is (of course) entirely true. With other factors beyond raw shader processing falling into the equation in any real-world gaming scenario, we never really see the full benefits of this increase in Stream Processors, but dependant on the title it does tend to offer somewhere in the realm of a 5-6% performance increase - Sometimes its more, sometimes its less, but overall of course this is nothing to be sniffed at when faced with the prospect of buying either part for basically the same price.

However, if you're already the owner of a GeForce GTX 260 with 192 Stream Processors, then such an increase isn't such a big deal, making trading up to the newer model an exercise that doesn't look particularly worthwhile. Indeed, the case for sticking with what you've got is even stronger when you start looking at these boards in terms of making games playable - At 1920x1200 with anti-aliasing and anisotropic filtering enabled yes, the Core 216 improves performance, but rarely does it do so at a level that will really make the difference between a playable and unplayable gaming experience.

Overall then, if you're buying a GeForce GTX 260 from new then there probably isn't too much to worry about here, as you should be able to pick out a Core 216 variant relatively easily thanks to most manufacturers differentiating their products. Even if you do own (or are buying second-hand or at a bargain price) a GeForce GTX 260 with only 192 Stream Processors, then you aren't particularly losing out on all that much, even when gaming at 1920x1200.