The GeForce 9800 GTX+ leaves the others behind at 1280x1024, probably because it has the highest frequency of the execution subunits (1836MHz) among all the cards in this category. For comparison, the execution domains of the GeForce GTX 260 Core 216 and GeForce GTX 280 are clocked at 1242 and 1296MHz, respectively. Judging by these data and the downright low performance of the Radeon HD 4850 X2, we can suppose that the game does not make use of shaders that require parallel execution and is quite satisfied with the resources available on the G92 architecture.
The new G200-based models of the GeForce series go ahead at the higher resolutions, and the GeForce 9800 GTX+ is unable to maintain an acceptable bottom speed at 1920x1200. Perhaps it needs more memory bandwidth or rasterization resources (it has only 16 raster back-ends as opposed to the GeForce GTX 200 series models’ 28 and 32).
ATI’s solutions are inferior to Nvidia’s cards in every mode. Even though the Radeon HD 4870 1GB looks good at resolutions up to 1680x1050 inclusive, it is inferior to its opponents in terms of bottom speed anyway. The Radeon HD 4850 is barely able to maintain a comfortable frame rate even at 1280x1024, obviously due to its low GPU frequency of only 625MHz.
We guess the GeForce GTX 260 Core 216 seems to be the optimal choice in this price category, especially in its newer version with a 55nm G200 core and lower power consumption. As for the cheaper cards, you may want to consider the GeForce 9800 GTX+, but the Radeon HD 4850 can be faster in other games. Therefore you should base your choice on the specific games you are going to play. You should not both about upgrading your GeForce GTX 280 because the GeForce GTX 285 is not much faster while the GeForce GTX 295 and Radeon HD 4870 X2 are considerably costlier.