Obviously I do not have huge experience with non-Europe long distance travels, thou I ride bikes for 15 years I can not say anything about other 2 choices, but my few cents about 650 and 800 DRs, mostly technical for what it worth.
I personally can not agree with one of opinions - DR650 will not do just everything DR800 do with same ease as well as DR800 will not do everything DR650 will with same ease. Weight/engine is a key to both scenarios.
Well set up DR800 will have almost same fuel consumption as 650. And DR800 have 24 liter tank. But still 650 will use marginally less fuel.
DR800 will be more of hard work in tight offroad than 650.
DR650 is quite less comfortable in long hauls than 800.
Seats just do not compare at all, size and riding position on big DR let you fall asleep while riding. A bit hard-ish seat.
Not much of wind trouble (of course comparing to 650).
On pavement 800 extra ccs allow you to cruise 140 kmph quite easily and even overtake hitting top speed of 160-165 kmph even loaded and with passenger or cruise with up to 120 kmph for whole day almost non-stop.
Off motorway DR650 will be easier in really tight twisty bits (weight, size).
For luggage capacity - well, DR800 won't notice 50 kg on the back

DR650 will, but not critical I think.
However, the most valid point is spare parts availability. Obviously DR650 would be easier on that when it comes to global approach. In Europe (all WE) parts for DR800 is totally no problem (any dealer of Suzuki and quite few stock parts shops will sell you parts for DR800), but other countries - can not tell.
I am a bit biased ending my own tests with buying DR800 as company to my sportbike and loving that bike more and more every day, specially after some offroad experiences, but I also had few test drives on both customized and stock 650s so I am trying to be objective. Also I would recommend to get SR43 (post-92) DR800 as per reasons mentioned in my reply in this tread:
http://www.horizonsunlimited.com/hub...-750-big-29350
one more thing - to answer - DR800 not watercooled, it is air/oil cooled.