I hate to instigate ST and Joker into another argument here, but I think people should know something about the way companies are starting to distribute their cores.
Back when AMD had the 3000+ and the 3200+ come out, they were in such a rush trying to get them to the market that they never had a chance to really test the cores until the demand started to slow down (which, by then many in the overclocking community had already discovered the extreme overclocking headway). Those cores were great, and very good overclockers, but some were only able to get 2.2 GHz to 2.4 while others reached insane 2.8 to even (though rarely) 3.0. Once the company was able to really test those cores, more of them became very similar in performance and overclocking.
There was a rush for the X1900's and especially with the 7900's (EVGA waiting list....yeah). I don't think it was so much the fact that the cores were tested and "not approved" for the respective title, instead I think it was because the cores were not ever tested to begin with. Both cores can reach a great levels when overclocking, but even though the cores (at least one) can withstand 1.575 volts doesn't mean that that should become the default voltage. The companies weren't going to have a GT, a GTX, and a GTXY, or an XT, XTX, XTXY. There always has to be overclocking room, otherwise it wouldn't get as many sales. Both ATI and Nvidia are making technology that can run very, very, very stable at their recommended speeds and even stable at much higher speeds. They simply made good cores and use two different methods of acheiving the "monster" within those cores.
Since the release of the X1900 series, ATI has had a lot of time to make this 1900gt, so I don't know if it they are just plain "bad" XTX cores with some reconfigurations to optimize its reduced level of fuction, or if they are just surplus cores altered to help the company sales in the midrange area (particularly against the 7900 GT). I guess I too am going to have to wait and see if they are just bios fixes and ATI's version of the 6800's unlockable goodies, or if they are carvings of the statue.