GDDR3 is based on DDR2 tech, so supports the transfer of up to 4 words every 2 clocks (ie 2 words of data per clock), whereas GDDR5 is based on DDR3 and supports the transfer of 8 words per 2 clocks, (so 4 words of data per cycle).  The actual word width per chip is the same, it simply allows the data to be fed in (and out) faster due to a large prefetch cache. That is so long as each transfer is from the same memory row, which is maybe what you're referring to. For video cards this is a not an issue as textures and geometry data are stored sequentially in massive arrays, whereas CPU's can be more random in their memory access. This problem affects GDDR3 as well.  However, this is something you can largely mitigate through cache and memory controller design, and through programming (ie keeping data access to 64 bit words only, not using x86's older, single/double byte access instructions).

GDDR5 also supports higher clock speeds through improved analogue design, so you'll see GDDR5 run at up to 1.75GHz (so 7 Gbit/s per pin effective clock) versus GDDR3 which tops out at ~1GHz (so 2 Gbit/s per pin effective clock). 

GDDR5 is a brute-force method to getting system bandwidth. Likely MS engineered with the high-bandwidth framebuffer EDRAM concept to keep costs down, as DDR3 is dirt cheap. I'd be interested in what the ramifcations of Sony specifying GDDR5 are, as to cost, power-consumption and heat-load. And also what sort of memory controller they are using.

The best news here is 8GB of whatever kind of RAM, this will avoid problems to fully tap HW power later in the consoles life and to port big games from other platforms.
Also, high capacity GDDR5 chips are still quite expensive NOW, but in 10 or more months they will have been already put in another several millions (or even tens million) devices, driving costs down. We aren't anymore in the times of late '90s - early '00s RAM crises, when producers made RAM prices wildly jump up and down, forcing BTW MS to halve XB1's initially planned 128MB RAM, and forcing games devs to quickly patch PC-XB multiplat games or to cripple them.

They can't just switch the ram, doing so would require a change to the CPU/GPU and their memory controllers, as well as a complete board redesign.

In the same way you can't just put DDR2 into a DDR3 motherboard.

Also. it's DDR3 and GDDR5, not DDR5, GDDR3 or any other incorrect format people are spouting.

LOL, this isn't as sleek as the PS3/Tefal Grill, but it looks a lot mightier!!!   

Doesn't matter, they were in the same generation.  Nintendo 64 was a 64-bit machine and Playstation was a 32-bit system, noone disagrees that they were in the same generation.  3DO and NeoGeo were both released during the 4th Console Generation and featured 4th Generation games (sprite based game, lacking polygon based models for the vast majority of their games).  Therefore, same generation as SNES and more powerful than it. 

Case rested, closed, and killed. 

Question: Why doesn't MS just have 16Gb of DDR3 I'm assuming they'd still only need two slots (2 x 8Gb ram) and it would still probably be cheaper than 8Gb of GDDR5. The other question I have is would 16Gb of DDR3 be better than 8Gb of GDDR5 ram?

someone needs to do some research, turbo graphics was way less powerful than the SNES and the genesis.  The genesis came out 2 years before the SNES, so it being slightly less powerful is actually impressive.  The SNES was not powerful for a 1991 console

the balls in sony's court. if microsoft want to change up the hardware now, it will cause delays and ps4 will get a head start and if they leave it the way it is than ps4 is more powerful and they both release same time. sony get an advantage both ways. unless those rumours were complete bullshit and xbox 720 is a beast.