everything is possible
1 now, I suggest you doing a dual boot for the only reason that it's faster.
2 but if you prefer staying on the usb you could always save your files on the hard disk
just create a folder on the hard disk and save all your files there
3 I am shore that you could also install programs from the usb to the hard disk, I've done it once while using backtrack 2 and it's not even so hard
just google it.
for any more questions, feel free to ask