fwiw, i have a x001 in one car and a x100 in another...
i've only had the x100 for a few weeks, so there may be a couple things i'm missing, but here goes... the only "big" ui difference is with the knob... when you push it, it acts as the "enter" button rather than the "search" button (which is nice), and that it rotates all the way around... there's a few other minor differences on the ui end, but nothing really major... ipod interface works the same... color is different... presets are still a pita... if you add the hd radio option, it allows you to "tag" songs for a later download (gimmick, imo, if i liked something enough to tag it, i think i could probably remember to get it later //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif )... there's probably a few more things, but i can't think of them off the top of my head...
fwiw, both are "sat ready"...
the big advantage of the x100 is the ability to use the imprint add-on module, if you want to go in that direction (i did)... if you aren't going to do this, kai's suggestion of getting the x001 and saving the money is the direction to go...
ymmv.
ps. 320gigs? wow. i have enough trouble remembering what is on my 80gig ipod.