Hi - I've just done an HDD replacement in my H320
the 40Gig drives may be getting hard to find - a 60Gig should be easy.
Start by looking here
http://www.rockbox.org/twiki/bin/view/Main/HardDriveReplacement for drive details.
Where you buy the drives from is up to you but good old eBay is good for searching if not buying. If you are in the UK I got mine from Ultratek's eBay store - I'm pretty sure they have 60Gig drives, but there are plenty of sites offering replacement drives for iPods - you need a Toshiba (only I think) and not one of the ZIF types.
Formatting the drives may need a utility (search the forums for swiss knife)
And if you haven't already taken your machine to bits go looking at Mistic River for instructions on changing the battery to find out how to get the player to bits.
Good Luck (shouldn't need it, it's not that scary)
Edit:
Mistic River dissassembly instructions here;
http://www.misticriver.net/showpost.php?p=363259&postcount=118please take a picture of the connection between the HDD and the mother board (I failed to and mine did not look like the one in the sequence I just linked to - I'm wondering if the later models might take the ZIF drives with a suitably manipulated cable...)