If you check the benchmarks and such at storagereview.com etc. you'd find that the 1200JB ROX! I believe it was something like 30% better (insane in the HD world) performance that the 75GXP. It's neck and neck with some 10k rpm SCSI drives!!!! That and the Ibm drives have questionable reliability...... (knock on wood, I have two 60GXP 40GB's in Raid 0)......
I plan on picking up two 1200JB's this summer to max out my onboard raid controller. which brings me to the second question....
I've got 80gb full now, plus a full 4.3gb system drive..... I have abot 5500 mp3's (25gb) and around 50 movies (30gb)....... That plus games and other crud....... and it's FULL!
I'd say to fill up 240gb, you'd want to get about 140 movies...... and a decent mp3 collection. Either that or lots of games and such. Just KEEP nearly everything you might ever use and you'll fill it up eventually..... then start deleting crap. I never thought 80 would get full...... but then I also remember people saying that "you'll never fill up a 1gb hard drive...."
Go for the 1200JB, it's only 25 bucks or so more than the standard 120gb drives, and it way faster.