There are two competing technologies: EDGE in GSM networks (DTAC, AIS, etc.) and 1xRTT in CDMA networks (CAT CDMA, Hutch, WinPhone, etc.).
The maximumum theoretical speed in EDGE is 384 kbps, however in the real GSM network environment the EDGE speed is much lower. EDGE in Thailand is available all over the Kingdom from DTAC, AIS and possibly TRUE. If your mobile phone supports EDGE you can use it anywhere in Thailand but it is quite costly.
I prefer to use 1xRTT technology in CAT CDMA network instead, the speed is very stable at 153 kbps, nevertheless the performance is similar to EDGE, sometimes even better. I have been using CAT CDMA in Nong Khai with excellent results. I believe this is the best value for money if you don't need to download huge files like streaming video and audio, it is quite adequate for e-mail, MSN and Skype chat, even for web browsing, the lattency is relatively small, websites are opening nearly as fast as with TOT ADSL connection. This 1xRTT connection is very reliable and stable, I can stay online for a few days non-stop and the connection is never dropped. The biggest advantage of this Internet connection from CAT CDMA is its cost: I am paying only 199.- Baht per month, this price generously includes 1,024 Mb of traffic (1 Gb).
There is a CAT CDMA office in Nong Khai. I paid only 99.- Baht to buy a SIM card, inserted this SIM card into my CDMA modem (which I bught in some small shop in Nong Khai for about 2,000.- Baht) and immediately I was able to use Internet anywhere in Isaan and some neighbouring areas of Laos including Vientiane.
Please see the screenshots of the speed tests.

