What version of Uboot are you using? Apparently newer version are recommended.

ATI Radeon X1950 pro 256 MB RV570 0.17 Working Sam460ex users are recommended to
use U-Boot 2010.06.04 or better
If that's not the problem I know Pampers had a Sam460 with X1950 so he should be able to advise on which card to use.