Yes, sli generally work but one card rendering the top "half" of the screen the other card rendering the bottom "half"
("half" because the dividing line is dynamic and shifts depending on the workload)
The performance "gain" will be very dependent on the game played, the settings and the resolution.
As I said the gtx460 1gb can currently cope with the most common high resolution monitors 1920x1080 (1080p) on the most demanding games atm, it's mainly when you look at the addition demands of 3d vision or multiple monitor setups when sli really comes into play.
Also some games run better with sli than others.
Currently games are outstripped by the performance of top end hardware.
I'm still on 1680x1050 like you and my gtx260 is not yet struggling at this resolution, with all the settings maxed.