Mac Pro 2008 running but no video out, is it a failing graphics card?
It seems that there have been problems with these cards and Apple is still fixing them.  I found this article about the ATI 2600 (http://www.itproportal.com/2011/02/21/apple-extends-warranty-mac-pro-video-cards/) which stays that Apple will replace the cards if they are a part of a bad batch they've identified.  I hope that helps.

-K