The best website for comparing electricity rates in Texas is the state sponsered
http://www.powertochoose.org/Be careful since some providers have either a $5.00 to $10.00 monthly service charge or will add the service charge when your usage drops below 500 KWH. The latter charge is not favorible for winter residents since you normally have little or no use during the spring and summer months.
You generally have to look at the Electricity Facts Label for each provider to determine total monthly charges. I usually survey the various providers about a month before my current contract is due to expire and select the lowest cost provider. I have had some issues with Ambit Energy and Gexa Energy. Currently with Spark Energy.
Update 10/11/2010..I just did a web search on "power to chose" and discovered that there are numerous {look alike} websites that are pretending to be the Texas Power To Choose site. Be sure to use the link I have posted here. It's a jungle out there and full of predators...