1

have taken buses between Udon Thani & Sakon Nakhon (roughly 3 h), & between Sakon Nakhon & Nakhon Phanom (roughly 1+ h)...so should be 4+ h total. comparing the info on old threads, don't think much would have changed for the buses:
http://www.lonelyplanet.com/thorntree/thread.jspa?threadID=2121866
http://www.lonelyplanet.com/thorntree/thread.jspa?threadID=1482754

2

The bus will take 5 hours at the minimum. I have taken this particular bus at least a dozen times, possibly a bit more. From Udon to Sakhon Nakhon will take about three hours, from Sakhon to Nakhom is 1.5 hours, add in at least half an hour at the Sakhon bus station (more if the bus is ahead of schedule). Take into account at least three smoke breaks and the bus stopping at each possible chance and you will understand why this bus takes so long.

At Nakhom Phanom, they have this new brdige, which is actually a good 8 km east of both NP and Tha Kaek, meaning the once easy boatride across the Mekhong now is replaced by 8 km going east, cross the bridge and another 8 Km going from the bridge back to Tha Kaek.

3

thanks to both of you for your useful replies :)