There was a limited number of Mach 1's built during its two year run. With every car that gets taken out due to wrecks or whatever, makes the remaining ones a little more valuable. Nobody can say with any authority what will happen a few years down the road, but the 03/04 Mach 1's stand a fairly good chance of being a collectible commodity at some point.
The Shelby GT500 is kind of an enigma to me. Although it has the Shelby name on it, it is produced entirely in-house by Ford/SVT, and Shelby Automobiles never touches it. Whereas the little Shelby GT actually passes through the Las Vegas assembly facility of Shelby Automobiles, so I would think it has a better chance of becoming a pure collectible down the road, but that's just my opinion.
Now the upcoming 08 Shelby GT500 KR and the Super Snake, which Shelby does the upgrades on, they will most definitely be collectible, and very expensive to get your hands on!