PS3 OS currently uses just 50Mb, I think theres a bit of breathing room with 512Mb
With how good the Sony engineers have been at shrinking the PS3 OS but continuing to add more to it, I'd be much happier with faster memory (and CPU/GPU ) over more. 4GB is much more than the PS3 with it's strangely split 512mb.
Or I will put it this way, we're not seeing more than 4GB of DDR5 as it'll give near another £500 PS3, which I hardly see Sony wanting this time around. I think it'll launch at £349/399 across two SKUs.
The RAM itself isn't split, Sony learnt their lesson with PS3, it will have one big 4Gb Unified chunk but 512Mb will be reserved for OS function.
There are unsubstantiated rumblings that Sony could free up RAM like a desktop PC does, it could unload everything unnecessary and just run whatever functions of the OS you would need while running a game, which could bring the OS limit down to 256Mb or possibly less.
I hate saying it, but the current rumours are that both consoles will be using GCN, Orbis will have 18GCN Compute Units, and Durango will have 12GCN Compute Units. 1.8TF compared to 1.2.
That's very troubling. I really hope Microsoft doesn't drop the ball with Durango. I assumed the GPU would be similar to the PS4's GPU (based on AMD's GCN architecture). It just doesn't make any sense for them to use something older like AMD's VLIW4 architecture (e.g. Radeon HD 6000 series).