I just bought the x1900xt 256mb and I must say Its great. I dunno how it fares against the x1950pro, But dont let something like the extra 50 in the name fool you into thinking thats its faster than the 1900. the x1900xt is a great card. I recommend it.
You are correct, the 1900XT should give better performance than the 1950, on paper anyways.. the 256mb XT rocks, had it been out when i bought my 1900XT 512mb i would have opted for it.