Wierd, the the $300 X800XL beats the $200 6600GT (barely), while the $200 6600GT craps on the $400 X800 Pro. ATI is really horrible this generation, especially with price/performance ratio. I wouldn't recommend going with any of the Radeon X series.
Did you not look at the benchmarks? The only one where the 6600GT comes close to competition to it is in Doom 3 at high settings at 1600x1200.
Now, that being said, the 6600GT is an 8 pipeline card, and the x800XL is a 16 pipeline card, so the two cannot be compared. The same can be said for those who want to compare the x800 Pro to the 6800GT.
6600GT = 9800 Pro/x700
6800 = x800XL
x800 Pro = N/A
x800XT = 6800GT
x850XT = 6800 Ultra (debatable)
Those, in my opinion, are the best comparisons you can make. Don't go blabbing about price or release date, I'm simply looking at cold, hard specifications.
Now in your case, the x800XL is my favourite card on the market right now. For a 16 pipeline card, the price is unbelievable. At the same time, for it's price, the 6600GT is a superb card. This can be summarized into the following:
6600GT - Borderline gaming card, good for those who want an affordable solution.
x800XL - a good midway solution between the midrange new generation cards and high new generation cards.