GDDR5 is much Much better than '3'. The speed itself is enough to convince someone of how better it is. If the speed is slow, its no use having 2GB. And, if your resolution is not that high, then its out of the question.
And since you said your resolution will not be above 1600x900.. You should go ahead with the 650M 1GB (GDDR5) Gfx card.