A newer hardrive will make a big difference in how fast your computer will feel. It needs a 50 pin scsi drive. I used to get these drives out of scrapped PowerMacs (250-500mb) and put the drives into my collection of Compact Macs. Works fine. I have a Performa 520 that I have upped the ram and replaced the drive in too. It is Running O.S. 7.5.3. I found a bunch of 1gb hardrives in an old IBM server. So far I have not had any luck getting these to work in my 520. The cd drive in the 520 has the removable tray, I think your 550 is like the 575. These drives are available with a number of different speeds. I would think if they plug in they would work. 2x-8x is the usual range that I have seen. System 7 is great to work with!!