I second the Matrox vote. They may not be the greatest performers for 3D, but then again most DAW apps don't need that stuff, BUT they are the most reliable and stable cards out there. Tech Support and driver updates are excellent as well.
I own an ATI and will never buy another one again. Their support simply SUCKS! It took them over a year after Win2k was launched to release final drivers and the Multimedia Center app for my All-in-Wonder 128. The extra features of my card (DVD,video capture, TV tuner) were all useless during that time in Win2k.
As far as Matrox, the G450 would suffice - there is no real performance gain with the G550 to warrant the extra money. The only difference is that the G550 supports some new "Head Casting" feature that is pretty much useless for most people.