I'll give these ?'s a shot:

Quote Originally Posted by bargaviking View Post
Dendrobatids seem like a fascinating group of frogs to observe.
Very true... I often find myself staring at my collection for hours on end trying to learn the behaviors of each frog...

Quote Originally Posted by bargaviking View Post
However, they seem fairly tricky to provide an environment for. I understand that they need a large amount of space relative to their size, but since most PDFs are very small, the enclosure still shouldn't end up being terribly big, should it?
I don't find it terribly difficult to care for them... Setting up the right environment does take time and a bit of research though... You'll want to figure out what species you'd like to keep first then build their enclosure to suit their specific needs. Depending on the frog and amount of that species you choose, you can have a vivarium as small as 10 gallons on up... For example, Dendrobates tinctorius are some of the largest dendrobatids but a pair can be kept successfully in a 10 gallon if set up properly.

Quote Originally Posted by bargaviking View Post
I am in college at the moment, and end up transporting my animals fairly often. With my frog's tank and my gecko's tank, there isn't a whole lot of space left in the car. Would it be all right to find a species of dart frog that would be happy in a smaller vivarium, and what species would that be probably?
Most of this answered above however, I wouldn't recommend PDFs if they are going to be moved around a lot... This can be very stressful and can have negative effects on the health & well being of your frog/s. how often are you going to be moving their enclosure?

Quote Originally Posted by bargaviking View Post
Would it be wise to have a breeding pair, would I be able to sell their babies to some kind of vendor?
I'd recommend starting off with froglets so you can learn all aspects of a frog's life cycle before trying to breed...

Quote Originally Posted by bargaviking View Post
Or should I just hold off on them (for a few years probably) until I can provide a permanent space for them?
That's going to be your call... If you can successfully take care of them and not harm them with constantly relocating them.

Quote Originally Posted by bargaviking View Post
Oh, another thing. Are there any dart frog species that do not call, or maybe the females do not call, or at least are very quiet? I am thinking D. auratus - the size is right, but I don't know whether they have a loud call or not.
I haven't worked with auratus but can tell you from experience that d. tinctorius have an almost silent call... Also, with almost all PDF species, females do not call.

Quote Originally Posted by bargaviking View Post
Thanks for your time. I'll probably have more questions after your replies.
Ask away!

-Christian