I have lived in BJ for 2 years, I would suggested the following choice of gear:
50mm, something really FAST!!!
What you will find in BJ is that the place is very congested, also when shooting on the streets, you will find a lot of electrical wires in the wrong places that will screw up your composition, so 35mm and wider will be difficult to use.
Also Hutongs, ChanganDaJie/TienAnMen area and the area around Hou Hai are very nice for photo shooting at night, I have had good results with a Takumar 50/1.4 lens for street scene with good results.
Wides
For wide angle shots in Forbidden City etc, I would say forget it and buy a post card, you will understand when you are actually there. And for going to the Greal wall, you really need something medium format, reason being that the great wall is not really that grand but it is really long, which also means it is very important to compress perspective. On the other hand, if you have tilt/shift lens, then bring it, you won't be disappointed. One more thing on the great wall, try to get a few people together and go to the wall at HuangHua (it is a bit of a trek but it was worth it!!! I have gone 3 times even though I am not a touristy type.), not only is it cheaper (you only pay the random farmer here and there), it is also a lot more authentic. Whatever you do don;t go to BaDaLing!!!!!!! I SAY AGAIN NO GO BADALING!!!!
Teles
A 85mm is can be useful, but I would suggest instead of bringing a megabuck lens, you should buy a Seagull TLR whilst loco ~300RMB/50US for the 75mm perspective, I thinks using a Seagull whilst in Beijing adds to the experiance.
By far the most important factor in photography in Beijing is the Choice of Camera, light conditions changes very rapidly, especally in the Hutongs, this is because the trees are in full leaf in summer and the shadows from buildings. You want something small, easy to change shutter speed, and inconspicious and quick to use. Light conditions changes at every corner and there are lots of corners!!
In summary, for Beijing, I would suggets the following kit:
For daytime:
35mm compact camera, small, inconspcious, quick to chages shutter speed, perferbly something suitable for zone focus.
TLR
For nightime:
Something that can mount a fast 50mm.
There is a camera market in beijing, about 1.5 km north of WuKeSong subway, on the right hand side just past the canal. Films are cheap and imported stuff are available at reasonable prices, as for camera gear, if you buy new, they sellers are fairly reasonable, good choice of russian optics tooo but check everything you buy throughtly. Stay away from the second hand gear at this market, I would say there is only one or two honest used camera sellers there.
And stay away from the friendly practise english people, especially in forbidden city and near HuangFuJiang. They also pray on local Chinese tourist so don't worry about being special.
I have hosted a lot of my friends coming to visit Beijing so you are welcome to PM me if you want to know more, I only left 3 months ago so my info are still quite current.
BTW my favourite photo of my time in beijing was a polaroid that I paid 10RMB to get taken, very kitsch but sums up the place perfectly!!
Good luck.