One more thing ,the PS3 memory is XDR at 3200 mhz instead of the 700mhz DDR2 of the X360.
Just one thing here. Main system memory of the PS3 is 256MB Rambus XDR at CPU die speed and can be accesed by the GPU via the CPU, just as you said.
Graphics memory is 256MB GDDR3 650 Mhz (1300 Mhz effective).
X360 has 512MB GDDR3 at 700 Mhz (1400 Mhz) that can be shared directly by the CPU and GPU.
The three current generation consoles have a 128 bit memory interface.
Just wanted to point that out