802.11a and 802.11b ARE NOT compatible with each other. Most of the networks out there are using 802.11b, so if you want to have fun wardriving then that's what you need. I'm sure you'd be able to find some 802.11a networks out there, just not as many.
802.11g is backwards compatible with 802.11b, and will reach the same speeds as 802.11a. However, it is not a finished standrad yet, and it WILL NOT work with 802.11a.
So basically it's one or the other, unless you can find hardware that supports both.