That'd be a minor, negligible upgrade.
The X1800XL is a great card at stock speeds, and since the first generation X1800XL's (hard to find by now though) are prototype X1800XT's, the Memory will over-clock to a tremendous amount, and the Core to an acceptable speed. That's because the Memory on the XL's is the exact same than that of the XT's (with only some differences in the circuits I believe, which makes the XL's Memory very similar to the XT, but needs more voltages, at least according to my experiences).
Mine has a 40 Mhz OC on the Core, and 130 Mhz OC on the Memory, and I pushed it at 175 Mhz OC once with more voltages, but it was running a bit hot for my own tastes (was 100% stable though). The X1800XL, once OC'ed like that performs very well, and my latest 3DMark05 score with it is around 8100'ish (7200'ish at stock). I really like it. And the 2nd generation XL's over-clock just as well on the Memory, and even better on the Core since they revised it for that generation and is stable over 550 Mhz Core (the first-gen XL's aren't).
The X1900GT is ok, of course. It's far from a bad card mate. But that card as an upgrade over a X1800XL isn't worth it at all. I'd rather save more money and go with X1900XT/X or X1950 Pro, or even more, just wait for the first-gen DX10 cards and see how they perform. Then once the DX10 cards are out, the current-gen GPUs' prices will go down and will be an even better deal to go with one of em'.