lets say i have 20 btc in my wallet.dat today. and i decide to back it up today and put it in a usb drive.
then three years from now, i have been racking up btc in my wallet.dat on my computer.
I now have 400 btc.
lets say i get a new computer and install the wallet.dat FROM the usb. the wallet.dat file is three years old.
will i have 20 btc in my wallet or 400?
The bitcoin client has the bad habbit of generating a new address every time you receive a payment. If you actually make sure you receive the payments to the same address all the time, and disable the generation of new addresses, then restoring your old wallet file should have the same 400 BTC. But you should make sure you store your bitcoins on the address that is in the original backed-up wallet.