Hi Sig,
A couple pointers - I think Bell Helmets used to have an ad campaign that said "If you've got a $10 head, buy a $10 helmet". I do agree with them. As tempting as I'm sure it is, don't buy a cheap helmet. I'd avoid injection molded helmets and stick with the kevlar/fiberglass models. Arai, Bell, Shoei make the nicest helmets and you can't go wrong with one of theirs. I've had some nasty head injuries (including breaking a very good quality helmet and sustaining a broken jaw) and I'm sure I'd be dead if I was wearing a cheap helmet in any of those crashes.
I also don't think that open-face helmets are a good idea if you're new to motorcycling, you're best off getting a full-face helmet that will keep your teeth intact if you do end up crashing face-first. But ultimately it's up to you - if you want to ride with an open-faced helmet, your chance of serious injury goes up.
The nice thing about open face helmets is that when you're travelling, it's nice for people to be able to see your face, especially when asking directions or crossing borders. Most people seem to feel intimidated (or insulted) when approched by someone whose face they can't see.
There are helmets I've seen that offer full-face protection but have a mouth piece that flips up to expose your face, and I think that they'd be great for a long adventure trip.
I'd also recommend a street helmet over a motocross helmet, since motocross helmets can get really loud when you ride fast, as they're not designed for keeping wind noise to a minimum. I ride with a motocross helmet and I wear earplugs because the wind gets so loud in my helmet that it would certainly cause hearing loss if I made a habit of riding without earplugs on the highway. Also, with a motocross helmet you'l have to wear goggles, which is an extra bit of gear you'll have to keep your eye on.
Hope this helps,
Wright
------------------
Wright Bagwell
http://fwc.webjump.com/xr650l/