I have a 40 gig hard drive in my player, with about 7 gig used - I have about 100 CD's on there! I usually rip at 128 bits, but even at 192 bits, I can't see you filling the disk up. 128 bits is CD quality and it's hard to tell the difference between MP3 and CD, especially whilst driving. You get Artist, Album and Track information, too.
Oh, and I have no financial interest in these players, but as the first 'consumer' of them in the UK, I would like to see them take off.