I would say it's pretty much worth it, especially because even if you don't like the upgrade (and you will) you have nothing to lose, since the Phenom II X4 CPUs are dirt cheap at the moment, and plus, you can upgrade to a new X6 CPU because it will be compatible with the current socket. There is no way PII will be slower in anything than C2D, that's for sure. However, you will not notice an upgrade in all things you do on your PC. For example, audio/video encoding and rendering, some games and many other software that is quad optimized will now run like a dream compared to your C2D, but casual games and normal PC use along with simple software will feel the same like before.
To sum it up, I think you can get a pretty decent deal on something like Phenom II 955. It will be a huge upgrade in some sectors, smaller in others. The real thing is that the platform you are getting is very versatile and it can be upgraded later.
So, you now want a good board. I suggest anything from Asus, like M4A79T Deluxe (the old classic, but still one of the best AM3 boards), or Crosshair III Formula as the best possible option. The CPUs you want to be looking at depending on your budget are Phenom II 965/955/945, or eventually the weaker 925 (it can hardly be found except in pre-builts).
Cheers and hope that helps.
PS: And of course, to warn you not to listen to the BS talk about PII being slower than C2D in dual optimized apps "clock for clock" (boy, how I like that phrase) as it simply isn't true. If anything you will see a tiny boost in dual optimized apps since the architecture of PII is superior, but it will really shine in multicore optimized applications.