I'm trying to fix up an old computer for a relative. I have a 20gb ata/66 and a 120gb ata/133 drive to use. Would there be any real speed difference if i put the OS on the 20gb, and used the bigger hd for data; or should i just use the bigger/faster hd for the system?

I would use the bigger drive for the system. You don't know how many more programs they might want to add at a later date. I have that problem myself. I have tons of storage space but the system drive is almost full.

I would advise you use the newer ATA/133 drive for the system, it'll definitely speed things up. However don't expect to get a sustained speed that's anywhere close to 133MB/s, I have a SATA drive as my system drive, even though I get an average burst speed of about 96MB/s, the sustained speed is about 68MB/s on the average. Pumpcast News

Does your motherboard support ATA100 or ATA133? If not, your ATA133 HDD will run at whatever speed the board supports (ATA66?). You will get better performance out of the 120GB HDD, even it it's forced to run at ATA66. Your 20GB HDD *probably* runs at 5400RPM (vs 7200 RPM for the 120GB HDD) & it probably has a smaller buffer. The 120GB probably has either 2MB or 8MB buffer. Also, the seek times will be faster on the 120GB HDD, especially if you setup a smaller partition (20GB?) for the OS.

jam's point of 5400 vs 7200 is the main one here. Also, 7200rpm drives run at a sustained speed of around 65mbs, making ATA66 not much different than ATA100 and higher anyway. I would just use the 120GB drive and use the 20GB as backup, partitioning the 120GB with a 20GB c: drive and a 100GB d: drive, then once the system is fully customized and all programs are installed use maxblast or some other disk cloning program to clone the c: drive onto the 20GB drive, so you have it as a bootable backup.