Don't forget that you're also trying to make a box you can sell for $299 and make it back in software (or even make a small profit). So you have three options:
Unified memory with no eDRAM (N64 and Xbox...both of which had bandwidth problems)
Unified memory + eDRAM (GC/Wii, PS2, X360...harder to master, always wishing you had more)
More, slower VRAM, but less main RAM (PS3...obvious limitations)
The PC route of huge pools of main RAM and VRAM really isn't option because of costs.
