While Halo 4 looked pretty good, it's obviously memory limited. Low resolution, low texture res, low framerate. More System RAM and GPU memory is required to raise all of those areas. Although, I agree 8GB in a console at the moment may seem excessive, but you have to realize these things have to last 5-8 years (and MS/Sony would much prefer 8 if possible.) Games on PC are already being 64 bit optimized (Says it right on the back of Dishonored.) Since the industry is heading towards 64 bit over the next few years it makes sense for both companies to take advantage of that for minimal cost. 4GB RAM costs you and I around $20, and 8GB around $35. Even if the price increase is the same for Sony and MS (doubt that, but let's go with numbers we have,) the industry is looking to make use of 8GB systems already... I think they'd side with futureproofing and spend a few extra bucks. CPU side is where they can slack. Even a low-end QuadCore will be able to handle pretty much anything over the next 5 years or so (hell a Q6600 with a minor OC to 3Ghz still works great today.) That's where they'll find most of their savings, and invest heavy in the GPU. Next round of consoles (the next NEXT gen,) will be something interesting as 4K TVs will be come the standard, and pushing those resolutions will be interesting to say the least.
In a PC you need more system memory in a games console you need more GDDR, system memory is not dumped into games on a games consoles it might be only $20 for 4GB of RAM but times that by however many units you need to shift and at either break even or profit margin it becomes an expensive waist, the most your going to see in the next Sony and MS iterations will be 2GB of system memory, while I agree with the lower res yes this is true but the Wii U exceeds the Xboxes 256MB by 768MB of GDDR5, if the next Sony or Xbox product exceed more than 2GB of GDDR5 then expect either a console sold at a loss or a console with much less CPU and GPU power than expected, 3GB GDDR5 cards can be in the region of £390-£450 and the 6GB GDDR5 monsters out there are nearly £600, Sony must have learned their lesson by now I highly doubt they will exceed that price point or want to sell at a heavy loss again.