Travel china and potential visa requirements

I plan to visit in October and November 2013. I'll fly from the USA into Hong Kong and then I'll visit SE Asia. After SE Asia I plan to return to Hong Kong before returning to the states. When I return to Hong Kong before flying back to the USA I was interested in visiting Beijing/Great Wall. My trip will total ~ 45 days however I will only be there for ~ 10 days of this trip. My itinerary is as follows:
1) USA to Hong Kong
2) Hong Kong to Cambodia
3) Cambodia to Thailand
4) Thailand to Vietnam
5) Vietnam to Hong Kong
6) Hong Kong to Beijing to Hong Kong
7) Hong Kong to USA

Do I need a tourist L and how many entries am I allowed into China/Hong Kong. I do not live close to an Embassy so I could leave out Beijing if needed