You will definitely want to do something to actively erase the drives before selling them. Simply deleting the logical volumes isn't sufficient. There are troubleshooting scenarios in the HP documentation which indicate that you can sometimes recover from an accidentally-deleted logical volume by re-creating it using the exact same configuration parameters. If nothing had modified the data on the platter yet, then the data from the old volume could be readable. It's unlikely that the purchaser of your system would set up a volume that happened to have the exact same configuration as yours, but it's still a real possibility that isn't worth risking (IMO).
If you bought the Smart Array Advanced Pack for your system (and if it's a G5 or newer), you will have access to a feature called "HP Drive Erase" that's designed for this very scenario. The controller will erase your drives by overwriting the data with random data patterns and with zeroes (you can select how many passes to make) and will optionally delete your volumes once it completes. This is probably the easiest way to go, although it's not necessarily the fastest.
If your particular drive model supports it, you can use the "Secure Erase" ATA command to instruct your disks to erase themselves (similar method via DOS instead of Linux). This is my favorite method because it is extremely thorough and relatively fast (it can be near-instantaneous on some SSDs). The downside is that there are still some older drives out there that don't support this command. If you go this route, delete your logical volumes before starting the erase operation.