I *highly* recommend the Garmin eTrex Vista Color, which I just purchased before spending a month in Cambodia and Thailand. The color is nice compared to previous models, but the big plus is the battery life. I've gotten over 40 hours out of two AA batteries (the pricy lithium type) which is nearly a week of full day use. I use the supplied lanyard and wear it around my neck so it's always handy. The Garmin world map (downloadable) is decent although not incredibly detailed. The Vista C has 24mb of map memory and putting all of Thailand, Cambodia and surrounding areas only consumed 3mb of that. Pick one up in the states for roughly $320 from www.gpscity.com
Re a using a GPS in North Thai / Lao - you wouldn't seriously use it for navigation, the maps are so bad. Only the main roads are on it & then they are sometimes kms out!
Generally there are no real trails or smaller roads, so I would not recommend buying one simply for navigation.
For me I use it for mapping & drawing my own maps.
On this board there are 4 other threads on GPS if you want to take a look & get some more info & ideas.