I'm going to start an adwords campaign soon, and wanted to know something.

Basically, I can get vouchers really really cheaply, like a $250 voucher for under $50. I know these have to be on new accounts, but my questions are:

1) Can I use multiple vouchers on my account? So, could I get 3 $250 vouchers and then activate them all for $750 worth of credit?
2) Do the vouchers expire once you put them on your account? So, do you have to use them by a certain time after you activate them?

Thanks in advance!