The 2900gt was a crap card for its time and was chopped when it came to vram. Check the card to make sure it is 512bit before you flash. If it is not then you are out of luck. The pro and the XT were fairly decent cards for their time while the gt was the bottom of the barrel like the 5830 is today.
so if its only 256bit then its a lost cause? just to let u all know i am currently using a 4870, the 2900 is just an old card laying around and i though that i might mod it just for fun if it was possible.