I went with an ATI card for my 51m for a few reasons:
1. Experience- I've had a desktop with an ATI card in it for almost 4 years now, and have had no problems with it. ATI's media center software isn't so good, but the hardware is pretty dependable. Also, my beloved GameCube is also ATI-powerd, and it has stood up to quite a beating as well (getting knocked off tables, being thrown around in checked luggage, etc).
2. I'm going with external wireless for the range, so I didn't have to worry about the hardware conflicts involved with an internal card.
3. I had heard that the Nvidia cards were in short supply, and I'm so dang impatient that I went for the ATI, hoping to speed construction and delivery
