256MB is still quite fine.
Some folks are hung up on more memory, but at this time the only real advantage is the ability to use ultra-high res texture sets available in a very small selection of titles.
Yes, the number of these titles will increase in the next year or so, but 256 MB will be sufficient for a while.
In the case of the X1950 XT and X1950 PRO, the XT's much-higher clock speed coupled with a much-more capable architecture trumps the relatively small advantage of an extra 256mb of texture memory.
At high resolutions, a 256mb X1950 XT will surge past an X1950 PRO with 512MB. I wouldn't be surprised if it showed double the framerates in some instances.
Aside from all that though: the 8800 GTS is quite a bit better than either...