Well you're right about them being made. Their just not available yet, read this review for some good info
http://firingsquad.com/hardware/ati_x800_x600_xt/ .
ASUS doesn't put anything different on their boards than any other mfg (other than PCB color and HSF mfg specific). There is a difference between an XT and a XT Platium Edition, about 20mhz I believe. All XT's come with VIVO (ati rage theater chip)
Which brand might depend on the bundled software and price. Each card preforms differently. ASUS usually has a good software bundle but is priced 20 to 50 dollars more than say a Powercolor with a limited bundle or why get the ASUS if you already have the software. ASUS does throw in a videocam as well...
Also don't buy a high end card if you're running a low end system. The card will never reach it's potential...