Hi again, Jackie.
Just a few things to keep in mind. If you are going to be riding into Chiang Khan on a weekend or holiday, make sure you have reserved a place to stay in. I usually stay at the Chiang Khan Guest House. It is right on the river and Pim, the owner, is great. It is a very bike-friendly place. Old school backpacking kind of place with gleaming teak floors and a lot of character. Thin walls, though, and shared bathrooms, but it attracts good, respectful folks. The link that Mike provided points out how busy Chiang Khan can become.
A great deal of the ride from NK to Tha Bo and then on to Si Chiangmai can be done on narrow roads immediately adjacent to the Mekong. If you go onto Google Maps and zoom in on the small villages of, for example, Pa Kho (about 10 km outside of Nong Khai) and then use Street View, you will see that these roads are excellent for riding, though not hardcore fast riding. They are concrete and usually in great shape and much, much nicer than riding on the main road. They also are nicely shaded in many places. In fact, you can avoid using #211 entirely from Nong Khai to just a little south of Si Chiangmai. Doing this will add a little extra mileage and time to your trip, but it is worth it. I was able to find all of these roads very easily with my Garmin GPS and SE Asia map card.
When leaving Nong Khai do not follow Highway #2. Stick to the road that follows the river (Soi Kaew Woravut). You will pass the old train station, go under the Friendship Bridge, and after a few km you will be out of town and getting into the villages. More road options ... the small riverside ones running through the villages ... will present themselves.
Tha Bo is a bit of an administration centre and there isn't a lot there to stop for, though the main market is decent. Si Chiangmai is a great spot to stop and get some spring rolls. The town is famous for it's spring roll wrappers. You will find lots of people selling them along the highway. When leaving Si Chiangmai stick to the esplanade that follows the river. It will take you out of town and is a fantastic ride that will eventually join up with #211 west of the town at a temple called Wipatsana Pa Chan.
Once you are on #211 the terrain will begin to roll more. If you take any of the minor roads leading off to the right, you will find yourself, again, close to the river and surrounded by fields of chilies, eggplant, and all kinds of other produce and, likely, surprised and smiling faces. Some of these roads can be a bit rough. A few were unpaved, hard-packed red clay that is very difficult to remove from clothing. Most are concrete, though.
Soon after Si Chiangmai #211 will begin running nice and close to the Mekong.
Before Sangkhom is a temple called Wat Hin Mak Peng, which is very well worth visiting. A great, shady spot to have a rest in. Very interesting place.
The ride into Sangkhom is wonderful. Great road conditions. And it just keeps getting better the further you go.
Have a great ride. When do you go?
Regards.