Click the link for the $240 agp 6800gt...lowest is actually over $300. Pricewatch isn't what it used to be.
I've never seen an agp x800xl in stock for $239. Lowest I can find currently is $270-280
I wouldn't compare the 6800 and the 6600gt, even if the latter is 256mb. IMO the 6800 will still out perform due to having more pp and vertex shaders. Also, the 6800 vanilla is generally superior, from a price/performance standpoint, to the x800 pro, from that I can find: $60-70 less expensive with slightly slower clocks and 1 fewer vertex shader, having also 3.0 support, for what that's worth
Please feel free to contradict me (with links) here as i'm actually researching the best $200 agp upgrade myself
