There are plenty of new ATA/133 and ATA/100 HD's around. An ATA/133 HD will work when connected to an ATA/100 HD controller on your motherboard (and vice versa).

Just get whatever ATA/100 or ATA/133 HD you like. Not much difference here. Just make sure it is at least 7200rpm and has at least 8MB cache. Make sure you use an 80 wire data cable rated for ATA/100/133 (usually included in the retail packaged HD).

Large hard drives over 137GB are a special concern on older computers. The computer's system BIOS and operating system must be compatible. More info: http://www.48bitlba.com

NOTE: Maxtor really pushed the ATA/133 hard drives. Seagage and Western Digital learned that they could essentially get the same performance out of ATA/100 HD's with sometimes less heat and noise. Again, not a big difference.