Hi InstImpres,
I'm the one who posted about the C315 camera. I just returned mine today actually...

The difference between Digital vs. Optical zoom:
Digital means that you can manipulate the picture on your computer before you actually print it out.
Optical means you can manipulate the actual camera zoom while you're taking the picture (you can be physically further away to the subject when taking the picture).
Without Optical zoom you will have to move closer or farther than the subject yourself instead of letting the camera do the work.
I hope I'm not confusing you.
Here's another explanation for it:
http://www.photoxels.com/article-optical-digital-zoom.html
As for what is a decent camera for non pro users, I'd recommend anything with 3 mp or above with at least 3x OPTICAL zoom.
The larger the Optical zoom the bigger the camera will have to be. If you look around the camera makers are selling up to 7 or 8 mp with just 3x optical and still have the camera be reasonably small in size.
However, remember that regular people will not be able to tell the difference between 3 mp picture and 7 mp picture when printed on a 4x6 or even 5x7. You will start to see some difference when you enlarge your picture above 8x10.