what gfx card?

hi i have bout 400 quid to spend ish give or take 50 quid so i mainly play css and i will be running a 20 inch monitor so 1600x1200 res what is my best bet as i have seen that the 1800xt is bout the same price as a normal 256mb gtx??
i dont want to spend too much so therefore the 512 gtx is to much, i have a sli board atm but if an ati is wuick enough or better buy then that will be fine ??

thanks in advance

My money would be spent on the GTX but that and the XT are very similar in performance