Since the cheapest XT I could find (after an admittedly brief search) is running about $335, for the first time in a long time I'd say go with this Sapphire retail XTX instead for $354 before $30 MIR... the XTX is worth this small premium over the XT.
http://www.newegg.com/Product/Product.asp?Item=N82E16814102037
Otherwise, the only real differences between the cards are warranty, Powercolor for example offers lifetime coverage on their retail cards, Asus offers 3 years as do a couple other companys, while the majority (including Built by ATI) carry only one year.
The HIS IceQ3 cards offer a much quieter & somwhat more effective cooling solution, similar to what the X1950XTX comes equiped with & also overclock very well for a small price premium.
Otherwise shop by price & unless your money is really burning a hole in your pocket, I'd suggest holding off just a little longer until the X1950's hit the shelves, as X1900 prices should drop more after that.