First off, as for "Not leaving well enough alone!" I'm the same, hence why I bought a half decent £400 machine a year ago and have now no original parts left and I'm sitting with a £1000 + machine. I also used old parts for 2 machines for my parents (I upgraded a few times
I'm currently on a Phenom II x4 965 3.4GHz BE. I switched from a Athlon II x2 2.9GHz. Performance wise I'm sure I would have noticed a greater impact because your Phenom I is probably a better CPU than my original, but you should notice the difference. A friend and I both had the same set up to begin with and the difference in games between the old Athlon II and the Phenom II is night and day. (Also, with the BE if you have the BIOS set to let it work full power, it will hit 3.6 without having to over clock it manually [which I know nothing about])
As for installing it, have you not done it before or are you just asking if it will be a simple swap? I installed a CPU for the first time some months back, I was sort of worried about it, but it was a breeze. I've done quite a few since in friends and family's machines and had my on out a few times installing new coolers etc.