Wont be much difference at 1280x1024, im stuck at that res with an x1800 XT, and i run everything maxed out absolutely fine, i don't see the need for an x1900 as the x1800 XT handles that res with ease.
Look a the firingsquad review in the link above, at 1280x1024 the x1900 XT 256mb beats the x1800 XT by only 2 frames in most games, only beats it by about 5 or so frames in about two, is it really worth the hastle of getting rid of the x1800 for peanuts, then putting some money towards what you got for it to get the x1900 XT 256mb when it only beats the x1800 XT in that res mostly by 2 frames, i dont think so, yes thats the 512mb x1800 there, but the 256mb is exactly the same.
You could wait for more opinions on it, but i think everyone would agree that at 1280x1024 its not worth going for the x1900 XT 256mb over the x1800.