Hey there, actually ram size, provided it's over 128MB's isn't going to make a difference in all but the most intensive and demanding games. While this would be fairly important if you were looking at two bleeding edge cards but, while great midrange cards, neither of these will benifit from 512MB of memory. The card will run out of steam well before extra video memory comes into play.
The GS has much lower clockspeeds than the GT version, other than that and the ram size (and speed, DDR3 being faster and more overclockable), they're essentually the same card. But the GT is easily superior due to vastly higher clockspeeds.