The thing is, those cards are maybe worth $20 in difference from each other in regards to street price, with the x1800xl being the more expensive part, obviously.
Would you see a difference going to an x1800xl? Depends what resolution/games you play, but for the most part yes. You might see a small difference regardless of resolution, but I'd say it's not even worth the hassle of trading up. Maybe an x1800xt or x1900xt is worth it, but not an x1800xl.