Beijing guesthouses are hostels & cheap hotels. in the week that i spent carousing around Beijing I didn't see any true guest houses. In some places the hostels and cheap hotels will be the same in all except name.

There were 3 areas that I saw where there were concentrations of cheap accommodations the main Beijing railway station which is located in the south east of the Peking subway Metro loop. The east side of Tianamen square and the south end of Tianamen Square which is called Qianmen. I found the Qianmen area to be the best bet there was a really cool shopping area which is the southern most of the green x and you could walk into the Forbidden city or hop on a lot of different buses or the Beijing metro which had a stop right there.

Best bet Qianmen Area 365 Inn, Sakura restaurant or Leo hostel both offered clean guest rooms at really cheap prices for Beijing and they had internet and food on premises. The fact that you could hook up with WiFi would be only reason that I could see to favor 365 Inn. This area is where I found the best Beijing cheap hotel as well
The Central Youth Hostel Near the railway station was one of the nicest hostels I have ever seen the only ones better would be the ones in Cairns Australia they also are a hotel so if you are looking for a discount hotel near the railway station I can recommend this one to you. A little tip is that if you are looking to buy Cheap flights to the rest of China the China Airline Office had the cheapest airfares in China, i wasn't planning on it but when I got the prices I bought my flight ticket her,e as I had checked the prices earlieron the internet and these were a lot cheaper by almost 40% I asked them why and the reason that I understood was because I was at the train station, and they want the tourists to fly, which was OK with me and my wallet.