I have had a good look at your site now.
The main problem you are having seems to be that you are using absolute positioning. Absolute positioning can cause all sorts of issues with layout as it removes content from the flow of the document. So if text flows out of the box, it will just keep spilling out (or dissapear if you are using overflow:hidden). This isn't too critical for the captions, as if you've got enough white space underneath them, they'll just fill that white space with higher font sizes or if you need to add more text than originally intended.
Your home page can cope with the absolute positioning, but the other pages are throwing a fit. If you increase the text size on your
Welcome Page, the text spills totally over the images below making it unreadable. If you avoid the use of absolute positioning and stick to using floated boxes you will have a lot more luck. If you do a google search for "CSS floats" you will find lots of information about floating boxes.
The main advantage of using floats here, is that as the text takes up more room, the box will expand. With absolute positioning, the boxes will stay the same size and the text spills out of the box.
For your menu mouseovers, take a look at
this demonstration on CSS mouseovers using a single image, they are much better than the hideous Javascript route that Dreamweaver uses for this technique. They work cross browser, are faster, are flicker free, are a smaller download size and work with Javascript turned off.
I'm not sure what type of "popup menu" was there originally, so I'm not sure I can help on that front. If you can show us what you are trying to achieve we may be able to help though.
If you do persist with the absolute positioning, you should make you've got a seperate box for each chunk of text, for example on your home page, the captions for two images are in one box. If they are in seperate boxes, you won't need to use 's to line them up and they'll still be directly underneath the captions regardless of what the user has their text size set to or what browser/font they are using.
Good luck with your design

Let us know if you need more help if my comments are too confusing (I tend to waffle a lot).