Any drive 120GB or less with a parallel ATA interface (e.g. ATA/100 or ATA/133) will work. I like Seagate because their long warranty (5 years, compared to others' 1 or 3 year warranties) hints at a higher-quality product. Seagates also tend to be quiet, and they don't cost any more than the others.
Note that getting to the hard disk in those iMacs is a bit tough. You will probably want to have someone experienced do it for you.
If you are running OS X, you will have to partition the drive so that you have one partition that is completely contained within the first 8GB of the disk, or else OS X won't be able to boot. If you are running Mac OS 8 or 9, you will not need to do this.
tooki