I would get the 512MB version, I have the X1950pro 512MB myself and I am very pleased that it is faster than the other 256MB versions on average FPS. My previous card was a 256MB card and would pause more often when loading textures in the games. The pauses in those same games, such as F.E.A.R., are less frequent and a lot shorter now with the 512MB card.
Plus I imagine that future games will be able to take advantage of more video RAM.
Same core, different bin. XTX is typically higher binned cores and clocked at higher speeds. You should be able to overclock the XT to XTX speed anyway, and get the same performance for less money.
x1950XT :
core: 625MHz
memory: 900MHz GDDR3
x1950XTX:
core: 650MHz
memory: 1000MHz GDDR4
If you're gonna buy a fast card, you're gonna run high resolutions with AA and AF, which typically eats up more VRAM. Seeing as the cards have similar clock speeds, go with the 512MB XT.