Avanti,
FAR 61.213 outlines the basic eligibility requirements. 61.217 tells you about the experience requirements to stay 'current.' I have heard of many people using the ground instructor ratings to pay for their flight training. Definitely get your GI. :-D
Once you take the FOI and GI (IGI or AGI - whichever your going for) take it to the FSDO. They'll do it, and often DPE's will take care of it for you. My best advice would be to take the ground instructor written the same time you take the written for the rating your working on...because it's pretty much the same test. i.e. - take the instrument, IGI, and even the CFII at the same time. You'll see some of the same questions on all three...of course, make sure you can get your CFII within 24 months. Would hate to see you waste 90 bucks or so if it expires.
Many people will tell you getting the ground instructor ratings is a waste. Personally, I feel the opposite. One of the best ways to learn about flying, is to explain it to others...whether in a ground school setting, or if your pointing to a blank wall talking to the couch. When you can verbally explain something, it'll make more sense then just reading it. The FOI will help teach you about the different learning styles. And also, having the GI's will allow you to go for your Gold Seal should you choose to do so.
Hope this helps some.
Cat