ExtremeTech has taken a look at the new GeForce GTX 260 Core 216 and ATI Radeon HD 4870 1GB graphics cards:
ATI's new $300 graphics card is easiest to explain—it's the same Radeon HD 4870 you know, only with 1GB of memory instead of 512MB. The clock speeds are exactly the same (750MHz core, 900MHz GDDR5 memory), the form factor is the same, and everything else is the same. It's just more memory, which should help most games that use a lot of memory for high-res textures and data as well as any game that runs at a very high resolution with anti-aliasing enabled (which dramatically increases the size of the frame buffer).
Nvidia's new $300 entry is a strange animal with a strange name. The original GeForce GTX 260 is a product with the same 1.4 billion transistor monster GPU found in the GeForce GTX 280, only with some of the functional blocks disabled. Call it a "salvage part" if you will, enabling Nvidia to use some of the chips that come off the line not fully functional in a lower-performing, less expensive part.
Check it out over here. The reviewer concludes it's not easy to pick a winner, if you play without AA the GeForce GTX 260 Core 16 is the best but if you enable 4xAA or 8xAA the Radeon HD 4870 usually wins by a nice margin. However, the reviewer notes the new NVIDIA card uses quite a bit less power and is less noisy than the Radeon HD 4870 1GB.