first, there are TONS of guides on the internet on how to upgrade hdd on a softmodded xbox...good "upgrade hdd xbox softmod" or something of the like.
Secondly, a couple of things you will need to know...
-you can have as big of a hdd as you like (up to 500 GB?)
-when you prepare the hdd using the linux tools (the guide will explain this) you will need to tell it to use F.
-you probably will need to re-do your softmod after you upgrade (because you may reactivate your orig. xbox dash).
- if your HDD is bigger than 150gb, after you reapply the softmod and are back up with unleash-x with your new hdd, you will need to go into the settings and tell it to "Enable G", then you will need to format G (can also be done from unleash-x). After you reboot, you should have all of your space of your new HD available to you, with F and G being your big partitions.