thx x1800xt are last-gen trump cards. so when the new cards came out (1900 and 7900 series..) the old ones decrease in price... however, some companies for any odd reason didn't decrease their old card prices.
get the cheapest one.. and preferably retail (get cables and stuff..might also come with games or something..)
i would recommend something like that.... i don't really know the difference b/w the 256 and the 512 version.. donno if its worth the extra $40 or so.. but its better anyways... for similar performance.. also look at the nVidia 7900gt