You cannot have 32nm AMD GPU since no such dedicated GPUs were ever made by AMD. It's either 28nm or 40nm. It's possible to have a 32nm APU but APUs have terrible graphics capability for next generation consoles. If PS4 only has an APU and no dedicated graphics, it's probably going to be underwhelming. Regarding going with x86 CPU for both the 720 and PS4, I am pretty sure that would kill backwards compability for both consoles. I see MS sticking with an IBM-based processor just because to the average console buyer "MOAR" cores would sound more impressive than 4 x86 CPU cores. I could see Sony going with x86 CPU because of how much negative feedback they got for the Cell processor. I think this time they are going to be smart enough to realize that a fast CPU is better than empty marketing claims. Since the Wii U development kits started appearing as early as April 2011, it makes sense why it's not using 28nm tech. Since we have had 28nm GPUs for a year now, 28nm GPUs in the PS4/Xbox 720 are 99.9% likely as those consoles' dev kits started appearing at a time when 28nm GPUs started arriving.

If either console is going to have a powerful dedicated GPU, there are not many options - mobile version of HD7870 desktop card seems like a good candidate (i.e., HD7950M/7970M). AMD just unveiled HD8000M but everything below HD8900M is too slow for powering PS4/720 (not powerful enough for a next gen console).

The problem is HD8900M is not due out until Q2 2013, although this does coincide nicely with the rumor that PS4's final specs will be finalized around summer 2013, which contradicts that claim that PS4 will be announced around May in the OP.

From the current crop of AMD's mobile GPUs, HD7870M is once again too slow as it only has 640 Stream Processors. HD7950M 2GB is probably a good estimate of the upper limit of GPU hardware due to its great balance of power consumption/high-end performance and likely a discounted price in 2013 as it won't be considered the latest and greatest.

If PS4/Xbox 720 pack an AMD GPU with more than 1280 Stream Processors, I would be pleasantly surprised as I can't see them trying to pull off a $500 console in this economy or fit in a GPU with 100W of power. I just hope PS4/Xbox 720 have some exciting exclusive games in the first 12 months following launch that can't be purchased on the PC. Thus far the Wii U has disappointed in this regard. 

is GDC the one in January where they revealed Vita?

Game Devolpers Conference takes place in march. 

Vita was announced in a japanese event called Playstation meeting.  I don't know if this event will happen in 2013.

That timeframe makes sense then.  Last of Us and Beyond will be fine sales wise even if PS4 is announced already.  I mean it's not like God of War 2 had any trouble (that was a sequel however).  They would almost have to announce it before E3 to clear the air for fully fledged details, Vita talk, and PS3 talk since there is only so much time in a conference, and they won't be able to keep that big of a secret anyway.  How awesome would it be if they did though?  I can't remember the last time I was actually surprised by a console announcement.

720 > PS4 >> Wii U

Both 720 and PS4 are 28nm or 32nm AMD GPU, 32nm AMD CPU. PS4 is comparable to existing "A10" APUs (4 x86 cores @ 3.5GHz), 720 has eDRAM on die as well and is much faster, both are custom chips though. The consoles are limited by 200W max power consumption so expect it to be midrange PC from a year ago kind of graphics.