I thought you had to purchase AppleCare within the 90-day complimentary period if you wanted to extend it (but I'm a Mac-newbie so I could be wrong).
As far as whether or not it's worthwhile, most swear by it but some say it's a waste. I think it comes down to your specific hardware and situation. A user who is switching from Windows might find it handy when figuring out how to do something in OS X. Also, with laptops, it's nice because they can be more difficult to work on.
I'd say it's not as useful with a desktop, especially something as easy to access as the G5 iMac. But then again the 20" iMac is ~$2k and AppleCare is only $119 (educational price). An extra 6% for that much additional support and coverage is quite reasonable.
As far as whether or not it's worthwhile, most swear by it but some say it's a waste. I think it comes down to your specific hardware and situation. A user who is switching from Windows might find it handy when figuring out how to do something in OS X. Also, with laptops, it's nice because they can be more difficult to work on.
I'd say it's not as useful with a desktop, especially something as easy to access as the G5 iMac. But then again the 20" iMac is ~$2k and AppleCare is only $119 (educational price). An extra 6% for that much additional support and coverage is quite reasonable.