With a month at your disposal, you should be able to take a nice gentle tour around Andalucia. Which places you concentrate on obviously depend on your interests:
Sevilla
Córdoba
Granada ( a must, for the Alhambra)
Walking in the mountains (stay low and close to the coast for the best temperatures in March)
Walking along the coasts and beaches
The Alpujarras and Cabo de Gata are especially special
This list could go on forever, so why not tell us a bit more about the sorts of things you like to see and do?
A car will be a must.
Regarding where to stay and how to book, I have an obvious commercial interest (see my profile) so I won't say too much.
Remember that the most touristy places will be pretty closed up in March. The best temperatures will be close to the coast and the higher you go and further from the sea, the colder it will get. (You could fit in some skiing in the Sierra Nevada.)
[ Edit: Edited on Jul 21, 2008, at 3:47 AM by LittleHote ]