It's not really hard at all, but if an inexperienced computer user tried to do it without any help at all, they would be sure to have troubles.
You can read data faster if it comes from the hard drive's bus - it takes longer for the drive to seek and read the data than to pull it as it gets read off the bus.. eh I have trouble explaining it... Anyways basically the drive will be quite a bit faster with the 8mb bus. WELL worth the slight ammount of money.
If you think you might have trouble with SATA (lets just say I dont feel like "tackling" it on my computer repairing/upgrading) I would just go with the IDE. It's not THAT Much slower. you will still have a nice, fast drive, so don't worry.