well, it's all your preference. Duo's are better for gaming right now, but Quads are more future proof.
yes, the GT is better than the original GTS, but there's the GTS with the G92 core, which is the newest of the 8800 series, if I'm correct. too.
here's a link. The GT is still cheaper though, and there's not TOO big of a difference, although there is one.
here's the best duo
here's the quad i was talking about