The E8400 would only be an issue in CPU intensive games.
Here's the win-win way to look at it: You buy the card and all of your games run faster (win), or you buy the card and see no difference-> you can carry over the new card to a new build (win) so no loss there.
Another thing to consider is your power supply. I've read
that the GTX 470 needs a 550W with 38A on the +12V rails and your 550W Gigabyte has 30A. Maybe consider one of the ATI HD58xxx cards that need less power.