I don't think Steam accepting bitcoin would have a significant effect on bitcoin prices. The primary demographic of Steam is probably very close to the same demographic of people that use bitcoin. This means that they would probably earn a lot of money by doing it, but they wouldn't really draw in many new people to bitcoin. This is contrary to, for example, Overstock.com, whose demographic consists of a much wider audience than Steam's.
I disagree with the assessment of the Steam demographic. I think the primary demographic lines up with -potential- users of Bitcoin, and those that are capable of using it in its current state.
However, of Steam users I know (and I am friends with probably 15, so this is largely anecdotal) none of them aside from myself have actually used Bitcoin for any sort of transaction.
Personally, Learning about Bitcoin was incredibly exciting, but I didn't become a real and true believer and user until I made my first few transactions and saw the simplicity of it. I don't think BTC offers an immediately noticeable advantage over other payment methods for Steam right now, so while I'm not convinced adding it and using it would have the same effect on others that it had for me, I
do think that actually using it is a crucial step towards individual acceptance.