The bottomline is that the price difference is more like $20,000 to $30,000 between the 06' Coupe w/low miles or a new 08' Viper Coupe. To me that is a lot of money.
If you can afford it, buy the 08' Viper coupe, as it is a much better vehicle and a lot of bang for the buck. If you watch you pennies, then buy the 06' Coupe and modify as you get bored with the factory power. There is nothing wrong with an 06' Coupe and it is proven mod ready which for some is great.
As long as you like it and are comfortable, that is all that matters. Other than tweaks they are still pretty similar, as everyone is treating them like they are a completely different cars. Com-on, it is really a new hood, new heads and some tweaked parts for reliability!
As an example, for me, I wanted a new Viper and I bought my 06' Coupe new back in 06' with a smoking deal. I can't afford a new 08' Coupe and why would I want to lose the initial money into mine just to keep up with the Jones' and lose on the initial investment in the 08' when it comes out. Lots of people can dip in $2,500 here or $5,000 there for little mods as time goes by. That is what bonuses are for, but initial investments are touchy, if you are financing this purchase, which there are a lot of people doing that. Just not enough differences for me between the 06' Coupe and the 08' Coupe to justify the costs. I mean a different clutch, difference diff, engine tweaks, heads, etc. for what will amount to $20,000 to $30,000 difference?
Buy the 06' (which has already taken the lion's share of the worse depreciation already), save your money and send you and your wife to driving school so you can get the most out of your Viper. I guarantee you will enjoy the 510 hp more in the 06' knowing how to really use it then 600 hp in an 08' without the aggressive driver training.
Just my two cents.