As far as I know, the Magellan series is your only option.
Garmins are nice for the areas that they cover, but they do not have ANY products, as far as I know, which have any Asia maps. So you would be stuck with the inadequate basemap for USA or Europe. You would get the country borders and maybe one or two major cities per country.
Magellan offers their WorldWide BaseMap product. It is likely to be a little sparse, but will certainly be better than nothing. You can expect many major roads, and probably many if not most cities and towns. It even has crude topo data.
http://www.magellangps.com/en/products/product.asp?PRODID=182This product will work with the Meridian series, or the mapping Sportrak units. I would recommend the Meridian Gold.