I think there are too many other factors, especially the fact that their introduction is some three years away, to come up with anything other than a wild-ass guess as to their impact.
Chances are that the early cars ('91-94 NSX coupe) won't be affected all that much. Look at how these cars have held up in price. It's really tough to find one for less than about $32K with moderate (say 50K) mileage, well-kept, clean title, manual transmission, no accident history. These prices aren't all that different from 3-4 years ago. They'll probably go down a few grand in price, but this would have happened anyway as the cars age. There are still lots of buyers waiting to enter the market at the $30K price point, and I suspect this will continue to be true.
As for the newer cars, look at what has happened to their prices. In the past year alone, the price of a nice '97 has dropped by about $10K during that period (from low-to-mid-sixties to low-to-mid-fifties), due to some combination of the economic recession, the incentives offered by Honda, and the intro of the facelifted '02. Arguably the economy may have been the biggest of those factors. In any case, this may be the biggest one-year hit in market value that the newer cars will ever face.
Regarding the new '05 NSX, we still have absolutely no idea of the two most important items that will determine the market value of the current models: the selling price, and the sales (number of units sold).
Bottom line: Three years is too far out to make any predictions. But I wouldn't bet on the new intro having as much impact on the market prices of the current models as other factors, like how the economy is doing, stock market prices, etc. And guessing at those other factors three years out is only that: guessing.
Conclusion: Don't worry about small differences in market price at some point in the distant future. Drive your NSX and enjoy it.
