Well these are tough questions. First, if you are having trouble coming up with a design plan I always suggest you drive around your neighborhood to see if anyone else has a design you like that might be amended to fit your home's needs. Plus, that way you get a good idea of what plants do well in your area. And, you'll probably see plenty of examples of what not to do. I'd take digital photos of the locations you like and start from there. I'd also check out as many garden magazines as you can (and photos on our website) for more ideas. Also, in some regions, you can often get help at a neighborhood garden center. Sometimes they'll even draw up a plan for you at no cost if you buy your plants from them. Check into your local resources.
As far as knowing when you have "too much" that's really a matter of personal taste. I guess the ultimate question you have to ask yourself is how much time do you have for maintenance. If you live a busy lifestyle, go for a clean, colorful look with low-care plants, mulches, etc. Don't try to do an elaborate English-style garden if you only have an hour or two a week to devote to it. I hope this helps.