Hexus has posted a shootout between two different flavors of X1900XT: one from Sapphire, & one from PowerColor. Seeing as how both of these boards sport the same chip, I'd guess the difference is in the bundle...

"In fact, there is no meaningful difference between the two partners' cards, and your choice, should you opt to go down the X1900 XT 512MB route for your PCI-Express gaming thrills, will come down to other factors such as price, bundle, and warranty."