how much does it mater to have 3 GB DDR 5 over 2 GB DDR 5 on video card
Depending on your setup, for just 1 monitor (1080p or 1200p) 2 GB DDR5 will be the best option, you won't have much of a difference on performance if you go for a 3gb card on a single monitor setup.
However if you look forward to game on eyefinity or surround (dual or triple monitor setup) some day, yes it does matter, and 3gb would be optimal for that.
If your going with any high power card ( 7950/GTX 670 and up) id go for the card with more memory as it will play a more critical role in the future, this isnt just because of higher resolution monitors becoming more popular, but also because of the Higher res textures, and effects ect, that are being added into game sbecause cards are now powerful enough to handel them.
Better frame buffer. 3-4 GB will probably be the industry standard in about a year, so it's probably a good idea to get a 3 gb card sooner rather than latter.