Id go with the 1GB just for that extra bit of memory, as for performance the 768mb version isn’t actually that far behind the 1GB.
I’ve had the GTX460 768Mb before and it was a good little card, it ran BC2 at 1680x1050 with ease and still got decent Fps, although in games like Metro 2033 it struggled.
If you can stretch your budget, consider getting something like a GTX560.