The RSX can be one of these things:
1. GeForce 7800 GTX (24 Fragment Processors/Pixel Shaders) running at 550Mhz.
2. A variation of the actual GeForce 7800 with 32 pipes and running at 550Mhz.
3. An actual GeForce 7800 with an "add-on" that is under NDA.
But the key of all the PS3 is the Cell, I believe that Sony has designed the machine for the postprocessing graphics in real time, this is why the huge bandwith between the Cell and the RSX exists.
In other words, the RSX can send the final image to the Cell or an uncomplete image for the application of an special image effect (like the Photoshop filters) that makes the final image that you see more accurate.
Since the Cell is more powerful than the Xenon we can see better graphics on the PS3, not for the PS3 GPU but for the combo of GPU (Render the image) and the CPU (makes better the image with post-processing).
Thanks, to all.