If the cards are equal, it will be better running two cards in SLI at x8,x8 versus one card at x16.
Now, if you are running say a SLI GTX260 or a single GTX470, the single GTX470 would be the route to go, so you have room to upgrade to SLI down the road. You might have some initial FPS lost (still very much playable) but once you add another GTX470 down the road, your performance would be much much better!
You are correct... SLI at x8/x8 for ANY card is always better than ANY single same card at x16 (you may find some older games that don't scale as well in SLI, so it isn't 100% of the time better).
However, I would recommend running just one GTX 470 and upgrade down the road to a second card, once performance decreases. When you are running an SLI setup from the start, it leaves you no upgrade path outside of purchasing new card(s)... IMO
Why is the feature to select 'most helpful answer' not showing up?
You have the thread setup as a discussion thread, which doesn't provide best answers. Your thread needs to be a question thread for you to be able to select a best answer.
If you want to select a best answer, just do a full edit of your original post and at the top you will see a tab to make it a question. Click on the tab, hit the submit button and your thread will be come a question thread and you can select a best answer from there.
Your best option is to PM a Moderator (Randomizer or 4ryan6 come to mind, which who I dealt with most for things) and request them to select which ever post you want as the best answer. You can PM a Moderator by clicking on their name on the main forum page and hit send private message.