I do know the GTX 275 is a better card. I suggest you buy that one unles you want to invest a little more in a HD5850 from ATI.
Buy the GTX 275.
As for why..that memory gain won't help that much, i do not know how expensive is the 1.8gb version or even if there is one..(i think there is one with a higher memory capacity from what i remember). GTX 275-faster in processing witch means better video quality and higher frames.
If you play up to 1920x1080 or 1920x1200 go with that card(gtx 275).
The choice is easy.