Oddly enough, I just tried all this out earlier today! Thanks to some of the great tips in this thread, I made short work of my assignment!
I noticed that I can export stuff out as a PSD too - but unfortunately, when I try to layer my speech bubbles into a Photoshop file like this, some strange distortion effect occurs. Easy solution to this was to do things the other way around - I ported the Photoshop file into Illustrator and flattened the image there before saving it as a 300 dpi TIFF file. For the needs of this project, that's just fine and dandy.
For some reason, if I try to export my bubbles as a TIFF file, I can't do this without taking the background with me. That's a pain, as if I could do everything with TIFFs, life would be super-peachy-easy.
Gotta say, though, after reviewing this thread, I managed to do all the required jobs pretty damned quickly. I'm definitely getting there, thanks to your advice.