O for the love of god, please go with matrox! ;D i have a millenmium g450 and rocks. Not to mention, have you looked are there parhelias??! <droooooooool>
they supply linux drivers, but i don't use them. i tried them on rh 7.2 and wasn't impressed. But the ones that X has are excellent. Also, another cool thing is that mplayer has a module for the matrox that uses hardware scaling to SIGNIFICANTLY reduce the load on the processor when playing DVDs.
another cool thing
btw, i think ATI has a dualheaded card, but Matrox is the better of the two in my mind