1

The border crossing north of Ejinaqi (in Western Inner Mongolia) is open, and Chinese and Mongolian citizens cross regularly, but I'm not sure whether or not foreign nationals are allowed across. If you do get across the border, you'll find public transport (buses) from the border to Ejinaqi, and morning buses from Ejinaqi heading east. The area is heavily militarized, and you won't be allowed on the direct Ejinaqi-Jiuquan road: to get to Gansu, you'll have to go a long way around.

I'm not sure if you can cross any further west (into Xinjiang), and indeed I'm not even sure if you can cross at Ejinaqi. The PSB here isn't a big fan of foreigners: I was kicked out of the town. The Gobi landscape around here is quite impressive though, and it's an interesting corner of China to end up in.

2

I can't provide you first-hand experience but this link could help you.

3

hey thanks alot for the great advice, it does indeed look like its possible to cross into china from west mongolia, which is great news!
any idea if its possible to get into tibet from the north?

4

any idea if its possible to get into tibet from the north?

No but as you have to use an agency to travel in Tibet they should be able to tell you.