High Resoultion Satellite Images with plotted Highway from the whole Yecheng-Kathmandu road:

Notes I made during my trip (they are based on http://www.adserballe.com/tibet2001notes.htm).

I messured the altitudes with my altimetre, so they are not that exact (for example the plains of the Aksai Chin region are about 100m higher, means 4800m - 5000m - but they are never all above 5000m, like most of the blogs you will find on the web are wrtitting about). The red numbers I just guessed to create the graph above. As a reverence value I noted down at some places the air pressure as well. The second altitude-coloumn are data I got from the srtm elevation model of the NASA. This numbers are a little bit more realiable as there shouldn't be a systematic error, but to messure the exact altitude from the model is not that easy, so there is a varritation of +/- 30m as I guess (it's most accurate in the plains).
The road conditons I often wrote down later, so it's not that accurate, also because it's very subjective. But you see, that over all, the roads are generally in a very bad state. However: as the Chinese are "developing" Tibet, there is maybe soon everywhere asphalt as I have seen it in the Gar-Valley after Ali, who knows. I heared it's planned to seal the road end-to-end untill 2008 (or 2010? - read here in german), after it is opend for foreign traffic in 2007... .