I was in Hanoi 3 weeks ago. I stayed at the all new The Drift Backpackers Hostel.
It's not really in the old quarter, just 10min walk from there, but I really enjoyed the walk as I found Hanoi a nice place to walk (comming from Bangkok everywhere is nicer to walk...)
The rooms and bathrooms are shared (1 bathroom with 2 showers per room, 8 people per room).
I can highly recommend this place. Havn't found anything similar in 2 months in the region. Last days I stayed there (nearly 2 weeks in total) the price was 5.50$. Not the cheapest you can find in Hanoi, but there you have free wifi on the room, free computers service, nice bar, movie room, etc etc and everything is clean.
Everything is new with western standards as they opened The Drift about 3 months ago.
And the staff is really nice, Tiff, Keneth, Thao, Nyung +++
Home
Also have a group on Facebook.
In Hanoi we bought 2 Minsk bikes (will be on sale in Saigon when we get there in 3 weeks). To go to Sa Pa it's a too long journey to do it in 1 day. Don't expect to do more than 300kms in a single day in Vietnam.
We leved from Hanoi too late and stopped 50kms after in Son Toy. The day after we started at 8:30 and reached Sa Pa at about 19:30.
In Vietnam 4x4 taxis and buses are
. Taxis are criminals on wheels and the buses are hard to overtake on twisty roads as they use every mm of tarmak and NEVER use the mirrors.
I really enjoyed the north of Vietnam, lucky you to travel there