If you are gaming on a 1080p monitor and have a Titan then you don't need SLI. I would only suggest SLI if you were using say a 760, and you wanted SLI for 3 monitor gaming or a 144Hz monitor. However, the Titan should max out your games already. No need for a second GPU. If you did want a 2 GPU system then you should get at least 750W but I'd suggest 850W. If you are using 2x760 then 750W is fine, but if you want 2x780 then 850W is safest.