At the risk of sounding a bit... 'snippy' - I love it when I read things like "Twice the performance" but can never seem to find any benchmarks that show 2x scores in score-based benchmarks or 2x fps in fps based benchmarks...
As for my opinion on the matter: I recently (about 5 days ago) jumped from a E8400 to the Phenom II x6 listed in my system below. I've had no complaints. Nothing was particularly wrong with the Core2Duo, but it had been a while since an upgrade and I was able to get a good price on the system. Even though the mega enthusiasts (
) here will suggest many different things based on their own bias (and I will do the same right now~), I don't see a problem with the upgrade you are considering.
I like the idea of more RAM... I really like the idea of a better motherboard with USB3 -- But honestly, I don't see any show-stoppers in your upgrade plan. - especially if the price is right~
Good Luck and Best Wishes with whatever you decide.