I'm betting the X1900GT is not a master card. You have to have an X1900 master card then another card, such as the X1900GT crossfire ready card, to go with it. If you already have a 1900 series card, you should return (or sell) one of the cards and get a X1900 Crossfire Master Card.
Core 2 Duo X6800 Conroe
Intel D975XBXLRK - 304
Thermaltake Big Typhoon HSF
2 GB Corsair Pro PC6400 4-4-4-15
74GB 10K Raptor HD
Soundblaster Audigy 2 SZ
NEC OEM DVD Burners (x2)
Gigabyte 3D Aurora Case
Ultra X-Finity 600W SLi Certified PSU
You need a "crossfire edition" card and a "crosfire ready" card. If your card has dual dvi outputs its crossfire ready. I know it is very confusing, I just went through this. Also, getting crossfire enabled is not as easy as 123. I had to download new drivers from ati just to get crossfire enabled. That was after following an exact sequence to get my computer even restart. Oh, and ATI is ZERO help. My screen still jitters when crossfire is on (only during games) and Ive spent 5 hours on the phone with ati. Good luck.