There are two main types of spring upgrades, a progressively rate spring, and a straight rate spring. A visual description of the difference can be seen in this image:
The stock front springs are 0.594 kg/mm straight spring rate. It is best for a very light rider, like under 120lbs. A common and one of the most highly regarded first upgrades for a GS500 is an increased front fork spring rate. The progressive springs are the least expensive option and a decent upgrade. Straight rate springs need to be tailored a little more to the specific rider and riding style. There are two main vendors of straight rate springs, Race Tech and Sonic Springs. There is a calculator on Sonic Spring's site for suggested spring rate given the rider's weight and motorcycle's weight: http://www.sonicsprings.com/catalog/calculate_spring_rate.php The owner of Sonic Springs happens to be a member on the GStwins forum.
The "lazy way" of changing the front springs can be found here: http://pantablo500.tripod.com/id2.html