Ive got a x1900XT and ive been thinking about watercooling too. One that i have found is from Koolance. Its the VID-200-L06 (Vid)
and it cools the memory too unlike some of the VGA waterblocks that ive seen. So this is one option.
btw, a better bang for your buck would have been an x1900xt, avg $100 less than XTX and will overclock to XTX speeds quite easily.