You need enough amps on the 12 volt rails of your power supply to power the card. An X2900XT needs something like 29 amps or so. Cheaper power supplies might say 500 watts, for example, but not have enough amps to power higher end cards. You should be able to find the amps listed on the sticker on your power supply.
RMA means returned merchandise agreement. You need to e-mail or call the card's tech support number to get an RMA # that you'll write on the outside of the card's package you mail back. They won't accept a card for warranty without an RMA # on the outside.