7's fine, Mac's fine. Personally, I'd go for the OS that supports the apps that make me money. But if you just want to stay in touch with the world, consider a netbook such as a Dell 9, 10 or 10V. If you want a cheap, netbook-like device but want OS X, consider a Dell 9 or 10V: OS X installs nicely (but not on the 10). Forget XP. It's wonderful, but it's history. There are also major-yet-niche brands out there: NEC make outstanding semi-rugged stuff but are useless at telling anyone about it. I'm on a Kohjinsha SC3 right now which is way worse practicality-wise than a netbook, but way more interesting.
Re "how much support will you get with W7 in the third world?": well, lots, actually. People don't get thicker the closer they are to 3 than 1.