Seriously, do you NEED to upgrade? You've probably got the X3 for 5 months max, and you want to spend another $250 for what?
Of course, it's your money and you cand do whatever you want with it, but wouldn't you prefer to get a better graphics card, or monitor, or something else?
Worst case senario if you add or change too many components windows might ask to reactivate, if that doesnt work by internet just call them on the phone and go through the automatic blablabla listing the numbers on the screen... works almost every time.
Motherboard changes is what forces anyone to reinstall... even at that I gotten away with not reinstalling windows switching motherboards thanks to similar nvidia chipsets...
If you have the spare cash and you want extra performance, I would recommend a high quality CPU Fan such as an Artic Cooler Freezer 64 pro and overclock that Phenom720
If you burn it as a worst case senario, you can get your new cpu and still have a kickass fan that will keep the cpu in the 30Degrees at stock speeds.