One thing to note is that most consoles are architecturally very dissimilar from PCs. The Xbox was a fairly normal PC, all told; but most of the others are something else entirely. A lot of them, for example, will do something like have a fairly small amount of high speed ram shared between the CPU and the GPU. At least these days, the actual RAM chips are pretty much the same everywhere, because it is a lot cheaper to use one of the well defined commodity RAM types (DDR, DDR2, GDDR3, etc.) than it is to define some sort of wacky custom RAM; but console architectures are pretty unusual by PC standards.