No it's not the RAM. Sure a memory bottleneck will limit performance but the reason why games run on a lower resolution, framerate, effects or a combination of the three is the lack of GPU grunt.
Its the RAM. Microsoft themselves (well an engineer) let it slip that a better APU would have been a waste anyways because it just be data starved and wasting clock cycles. A better GPU would just sit and be waiting for data unless they increased the size of the eSRAM dramatically, or went with faster main memory than DDR3.
People need to stop the malarkey and accept the hardware tradeoffs MS thought prudent for their "Vision". Its not like this info wasn't out there since E3 (and before).