The OCZ StealthXtream 500W PSU is an excellent power supply. It's very reliable as has plenty of power. It should run two 6800 GS cards quite well.
The video cards used in SLI do not have the be the same brand, but they both have to have the same GPU. So as long as you get another 6800 GS to match your current one, you'l be fine. You'll also want to make sure they both have the same amount of memory. Otherwise you'll need to force both of the cards to use the lower of the two memory sizes and operate together in SLI mode.
Here's everything you need to know about SLI: SLI Zone - FAQ