Yes, you can delete the data you don't need. It's okay to do this on your computer if you know what you're doing...just be sure to delete all the files related to a given state or chart. And yes, you can use an 8gb SD if you wish...but if you have any problems with your iFly after switching SD cards switch back to the one we provided. Sandisk is a very high quality card, and cheaper cards can cause sporadic problems.
-Walter