I've built many computers before so I think I have a clue what I'm talking about
I would really wait just a month or two until nvidia releases the next series in their gpu line up. ATI has already released their new gpu series and nvidia will follow very quickly.
As for motherboard upgrades, it will better to upgrade your motherboard and CPU at the same time. That could end up being more expensive than the gpu replacement and your just going to end up upgrading both things probably within the year.
I would wait a few months, intel will release more of their new CPUs and they will be much lower in price.
I'm on he same boat as you. I currently have a 480 gtx and a x58 motherboard with a i7 920 at 4 ghz
I need to upgrade my gpu and want to upgrade my board and CPU but I'm playing the waiting game for a bit