4 GB of RAM seems short sighted on Sony's part. What's the point of having the better graphics proc if you don't have the mem to hold the textures? Also, 4GB RAM is the rec spec for games being released NOW. If they're expecting this console to have a 10 year cycle, 8MB give you so much more room to grow.
Also, the difference between the GPUs won't make much difference in game graphics. Devs will just code for the XBox power envelope as the LCD, and it will basically look the same on the PS.
I hope that MS really takes the RRoD lesson to heart and implements proper cooling in their console this time. That might also explain why they're going with a midrange GPU instead of a high-end one.