I know for a fact that I can overclock my current card without issues. I was under the assumption that for sli to work correctly I needed the same card at the same clock speed. Should it work better at stock speeds or better with the cards matched? I'm just looking for some reassurance that it will work fine before I waste $200 on a card instead of just doing an upgrade to a 600 series card.
Edit: Found my answer 2 questions down http://www.tomshardware.com/forum/394084-33-adding
Thanks for pointing me in the right direction maziar