Understand that all things being equal, GDDR5 has the possibility of performing better than GDDR3. But on different GPU's, all things are NOT equal, and the type of memory used is no indication at all of the cards overall performance level. You need to look at some benchmark charts of how the card actually performs running the games or programs you intend to use, that is the only way you can determine performance level or differences in cards. And since you have a couple of threads going here on this same subject, I'll add that all PCIe type cards are backwards compatible with all PCIe slot versions. Do not worry about that either, just pick the card that performs as well as possible within your budget. Use the GPU charts here on Toms Hardware to help you with your selection, and also check out the monthly "Best GPU for the money" articles, they are pretty good to get a overall impression of a cards performance level at particular price points.