Hi, ref memory speeds, the original SDR RAM could only transfer once per clock cycle, hence the term retrospectively applied, Single Data Rate.
In the quest for memory bandwidth they came up with DDR, Double Data Rate, that can make two data transfers per clock cycle, once on the rising clock signal and once on the falling.
As I understand it GDDR2, GDDR3 and GDDR4 are more-or-less faster standards for DDR, same idea and operation just revised timing / voltage to allow faster operation.
I think GDDR5 is able to transfer 4 times per clock, I.E. 900MHz base clocked ram will be advertised as 3600MHz. This will give GDDR5 twice to memory bandwidth for a given baseclock/interface width over GDDR2/3/4. Do often need to check what the given memory frequency is, if is the base clock or the effective clock, usually they give the effective clock as bigger numbers = better!
There is a table on this page on Anandtech
that lists some cards with interface widths, memory clocks and effective clocks and the bandwidth this translates to, might help.