Theinquirer.net can report some interesting tidbits about the PS3 GPU after a recent trip to Japan. It seems like the second gen dev kits were running nowhere close to full speed or spec.
Documents shown to them on the trip showed that there were two versions, labeled 'CEB' and 'DEH'. The earlier CEB has a chip called CBE connected to main memory and to a south bridge. The SB is then connected to a chip labled 'NV4x' over a PCIe link. The upcoming version DEH has the CBE and main memory, but the NV4x/SB combo is replaced by the RSX. If you wanted to know what makes RSX more than a GPU, it looks like it is SB functionality.
The slightly more curious bit, encouraging and disturbing at the same time, is the current state of RSX. The disturbing part is that the slide I was shown had "Current DEH's aren't final spec or speed" in bold letters. Speed, OK, but not final spec at this point in time leaves precious little room for debugging before the console release. On a different note the current ones are running the RSX core at 420MHz with 550 expected for launch. Memory is set at 600MHz with 700 hoped for as final.