Hi RS4,
I purchased my S5 from Maine (I'm in Texas). It is tricky and totally dependent on the state from which you buy the car. I purchased from an individual, but I had to fork over motor vehicle tax here in Texas when I purchased the car. Here is an example though to help you understand how messed up the taxation from state-to-state is:
In Arizona (I use Arizona because I almost bought an S5 there first), they will generally charge you their own state tax rate if you buy a car from them and you live in CA, CT, FL, ID, IL, IN, KY, MI, MN, NV, NJ, PA, RI, TN, TX, VT, or WA
Because these states have higher tax rates than Arizona’s 5.6% transaction "privilege tax rate", motor vehicle sales to nonresidents from these states are subject to Arizona taxes if the nonresident purchaser takes possession of the vehicle in Arizona. These states will allow a credit for the state tax paid in Arizona.
A so called "non-resident" that buy in AZ but lives in AR, DE, D.C., MD, MS, OK, SD, or WV are exempt from the AZ tax but generally have to workout the tax with their own state.
The rest of the states in the US pay variable tax rates ranging from 0% to 6%.
Bad news is that a lot of states have screwy tax setups for cars purchased out of state. I would call the local tax office (where you register cars) in the city from which you wish to purchase your car and ask them if they have a non-resident tax rate for motor vehicle purchase.
They most likely ask which state you are from and (fingers crossed) maybe you'll be exempt. If you are buying from an individual, like I did...then toss all of this out of the window and be prepared to pay your state motor vehicle tax at the close of your deal or when you register...just in case...
Good luck!