I've been looking pretty hard at power supplies for the $2k system, and honestly I can't find anything close to as good a value as that Antec TrupowerII 550w. It's got 38A total on the +12V rails, which is hard enough to find until you get to power supplies that are $100 more expensive. It's quiet, stable, SLI certified, and I also own one and have installed several of them without ever having a problem.
People are certainly welcome to substitute if they like another brand better...but I think I'm going to stick with that one in the $2k system. Everything I've seen on the X1900's power draw is that it's somewhat higher than a 7800GTX, but not that much higher. ATI recommends 30amps on the +12V rails (they say that is what's needed for a fully loaded system, not just for the video card), this power supply has 38. If someone understands power supplies a little better than I do, I'd love to hear exactly how the dual +12V rails work, and whether adding the amperage together (like they do for SLI certification) is the right way to go.
Most other reviews/info I've seen about the X1900XTX (which draws more power than the XT version I'm recommending) say 22 to 25 amps on the 12V rail is safe for a single card.