George--If the cassette and chain are in good shape run them. If they show signs of excessive wear replace them. Look at the gears on the cassette that you use the most often. Then compare it to the gears you use the least often. Does the drive side of the teeth show a cup like or dished out wear pattern? Are the teeth pointed on the ends instead of blunt? If these conditions exist the gears are worn out and should be replaced. I usually measure the chain length for wear and then bend it a bit from side to side to check for side plate wear. If it looks good, clean it up, and lube it and run it some more. My LX front XT rear equipped bike has between 7 and 10 thousand miles on it and shows only minor wear on the original cassette and chainring. I recently installed the 3rd chain.
Pretty flat over there in Katy. Cant imagine you needing a lower gear unless you go visiting in the Hill Country.
If you decide to go lower in the rear (11/34) you will probably have to replace the derailleur also. If your front derailleur is a clamp on it should work with mountain bike chainrings after being lowered to the proper height and then adjusted for proper side to side movement. You should be geared plenty low enough with mountain bike chainrings and your current cassette.
I have pretty much the same terrain and high headwind here in corpus christi as you do in katy and I almost never go lower than about 34 gear inches or higher than 100.
Look here for a calculater to help learn more about how gears work.
http://www.sheldonbrown.com/gears/