check out misticriver a great resource for all sorts of iriver mods, including a lot of threads on hdd replacements for the h100 series etc.
As ericj mentioned though, the largest hdd for a h120 is 80 gb, there is a 120gb that will fit inside a h140.
the procedure itself is dead easy, you will need a fat32 formatting app, depending on your pc and os I find fat32formatter to be more stable and reliable than swissknife if using vista or xp for that matter.
h120 firmware hex file
its essentially a case of opening the player up, removing the old hdd, fitting an ide to zif adaptor available on ebay/dealextreme etc for a couple of bucks (there is a newer smaller adaptor available which may prove useful, details can be found over on mistic river), attaching new hdd, connecting to pc, formatting to fat32, flashing the hex file and installing rockbox if desired and your good to go.
Fuller instructions are over at mistic river but its a pretty straightforward procedure and one which pretty much everyone should be able to follow without too much stress or worry.
gl