Everything is stored on the HDDs, so once you remove them the rest of the PC will become an empty shell. Only thing left that's accessible will be the BIOS, so no need to wipe anything else once the HDDs are gone. Don't need to do anything before you take them out either. Maybe de-dusting the whole will be best though before you sell 
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You can buy HDD docks that can connect to USB: http://www.overclockers.co.uk/showproduct.php?prodid=HD-015-AK&groupid=701&catid=14&subcat=72
Then you can use the HDDs as an external or new internal drives if you plan to have a new desktop in the future.
.You can buy HDD docks that can connect to USB: http://www.overclockers.co.uk/showproduct.php?prodid=HD-015-AK&groupid=701&catid=14&subcat=72
Then you can use the HDDs as an external or new internal drives if you plan to have a new desktop in the future.