Yes, I replaced my original HD with a 100gig 7200rpm Seagate. It's actually very easy as long as you're adept at installing your Operating System of choice (Windows XP Pro in my case) and have a complete collection of drivers burned to a CD-R.
Just pop the HD out, remove it from it's little case-frame, set the new one in and you're good to go. Put in a bootable CD in the CDROM and you're all set.
The only problem I have is now, my computer is slightly unreliable coming out of Hibernate mode; which is more likely something to do with my drivers. 1 out of 4 times, when coming out of Hibernate, the progress bar reaches the end of the screen and the machine just hangs until power-off and reboot. It never has any problem on cold-boot.
Performance wise, the faster speed drives DO make a nice difference.
I've also bumped my total RAM to 1gig as well by replacing the "behind the keyboard" stick. That was trickier than it sounds.
Maybe someday I'll glue down the monitor cable and stop the flickering as well...