see this URL for current seat map.
http://www.seatexpert.com/_AA_767_300_NGBC.html
ALL 767-300s now have this layout.
you do NOT want row 20 under any circumstances!!! they don't recline and on a long flight you won't be able to sleep, or even be comfortable sitting up, IMHO. There are worse seats, e.g. the very last row next to the lav, but row 20 is pretty bad.
If you are able to move to row 21, you'll have lots of legroom, but be sure to get an extra blanket, as it is (as seatexpert and seatguru say) usually quite cold next to the exit door/window.