I have a 3.06, and I'm pretty happy with it.
However, now that I think about it, an i5 would have lasted me longer.
But anyway, at this point, you might wanna consider going with the i5, it'll just last you longer, and it's the new thing that can handle more work.
You may not notice the difference now, but in a few years you will.
i5s are being put into all new computers, so it just looks like the right thing to do.
Personally, I run mostly all Apple apps that came with my computer, and so far, it has never slowed down even a little. However, I do have apps running in background at all times. Stuff like Mail, iChat, iTunes, and Safari are always open, and it seems to be working fine for me. Occasionally I'll open Aperture and Photoshop, which also run smoothly.
Anyway, unless you need to do some heavy duty work at this moment, go with the C2D, but if you're planning on using this computer for a long time, and wanna stay relatively up to date in about a 2 years, go with the i5, or even better, i7.