Obviously no one really knows, and what actually happens will be driven by demand for the Gen II cars once Gen III arrives. What is working in your favor is that you are looking at some form of GTS. Here are some observations:
(1) The GTS will not be produced again until 2005 at the very earliest (other than the Competition Coupe, which isn't a street car and is ultra-low-production, so is not germane to this discussion). Therefore, anyone wanting a coupe, which includes most everyone who wants to road race the car, those who like the coupe styling better, those who don't want the RT due to the climate they live in, etc., will have to buy a Gen II car. This should cause some increase in demand for Gen II's and therefore cause them not to depreciate as rapidly as they might otherwise.
(2) Based on polls I've seen around here, people who prefer Gen III styling are in the minority. Therefore, people looking for a Viper for styling will tend to gravitate more towards a used Gen II. This should help keep demand up.
(3) From my highly biased perspective, the Gen II GTS is one of the most beautiful automotive designs ever, especially the blue with white stripes and yellow (oh, did I mention this is biased
)? Surely there are others out there who think like me who would want to buy your car!
All that being said, if you slap a nitrous kit on the car, and drive the crap out of it on the track every weekend, or put 50K miles on it, you will lose a lot of value, GTS or not!