some sli advice please

i currently have one evga gts 250 superclocked i just recently got a job and got a big first pay for the training, now, i wanna oerhaul my rig, my frist problem is sli. would sliign my rig make a difference? i hear it does but what kind of difference are we talking? 5-10 fps? or more significant?

Last I saw, with the 200 series, SLI gives about a 50-60% boost in power. For the cost, I think you would probably get more of a performance increase by upgrading to a GTX260.