I was afraid of lower resale value when i had my 01 GT due to the 05 comming out. In the end i bought my GTO a couple of months ago and got positive equity from my Mustang. The 99-04 Mustang will retain excellent value if its a V8. A lot of people aren't into the new retro design and are willing to get an older model instead, at a lower price. So demand for your car isn't going away. The Mustang is a popular car no matter the year and people will buy it. I wouldn't worry to much. The best thing you can do is get a quote on your car now. Clean the car as much as possible and bring it down to the dealer. Make sure you already haggled a price for the 05, i wouldn't pay above MSRP otherwise take a walk. If you already know the price on the 05 then negotiate a trade in price on your 02. Go to different dealers and do the same. Use Edmunds and KBB as guidlines to what your car is worth and what they can expect to sell it as.
REMEMBER TO HAVE ALL OF YOUR NUMBERS WORKED OUT BEFORE YOU GO TO THE DEALER!!! Know what you are willing to pay and willing to accept on the trade in. Don't take anyones word. Your trade in is almost guaranteed profit for the dealer. Its really a matter of how much profit they will get from your end as they already know how much they will sell the car for. Don't get caught up in the excitment of buying a brand spanking new 2005 Mustang GT until you are happy with the deal that is ready for your signature. Either way if you wait now, the resale of your car will go down, but now by much. The year of the car isn't as important as you think. Yeah a 20 year old car isn't worth much, but a 3 year old car with 20K miles on it and a 2 year old car with 30K miles on it would almost have the same value if in the same condition. Mileage, presentablility, mechanical and cosmetics are what they look for. The better looking you car is the less work they need to do to prepare it for sale. In the end wait if the price isn't right for you, go for it if you get a great deal. Never look back as someone else will always get it for less.