I use both rheostats and thermostats with equal success, but my local climate is extremely constant (ranging from 50-60 all year). If you have a more variable climate, I would suggest a thermostat for full accuracy - they're better anyway, but I didn't want to buy four $150 thermostats when a $10 dimmer works fine here.

You can use one device for both racks, but I've noticed they work much harder that way... I have to keep my herpstat set at 97.5, which barely keeps the hot spots at 85-87 on the two racks it controls. As for buying the flexwatt, here's the place to do that: http://www.reptilebasics.com/flexwatt-heat-tape/ (you can even have it pre-wired for an extra $5 )