if you search for "fork buying guide" you find:
Fork buying guide:
http://bikeforums.net/showthread.php?t=105395
if the bike is the 1990 rockhopper in your sig, I'm in a really similar boat. my dad has a 90's hardrock that he wants a suspension fork for. as far as i know, with a bike that old, you are limited to a quill stem, or "threaded" fork. there are adapters to let threaded forks use threadless stems, but no adapters to let threadless forks be used on threaded bikes. so that will greatly reduce your search results. then you have to measure the width of your fork's steerer tube, and the length. it has to be the right width because a fork too big won't fit and a fork too small won't be secure. the length just has to be the perfect length or longer, if your steerer tube is 5.5" you can get a fork with a 8" steerer tube and just cut it. it has to be a very flat and exact cut, so if you are nervous about doing it right just have your LBS do it.
if you are doing this yourself i like this site:
http://www.utahmountainbiking.com/fix/index.htm
or
www.parktools.com
or get yourself a nice manual like Zinn's bike repair.
to help decide if you should get a new ride, it depends on what you intend to do. if you wanna use the bike just for light trails and road, and just want the fork so that it's easier on the curbs, roots, etc... and the bike fits great and runs great and you really like it, then just get the fork. but if you want to upgrade the bike because you want to move on to bigger and better things, and could picture yourself outgrowing that bike even with the new fork, then go for the new ride. Also I'd recommend not spending perhaps more than 50% of the bike's value on parts and labor. a '90 rh comp is worth perhaps $100 - $150 so i'd spend no more than $50 to $75, more or less depending on how much you like the bike etc...