Optical drive
If you're planning on installing Windows 9x or have some old games on CD lying around), you of course need a CD-ROM drive. If you don't have anything on CD and don't plan on buying any, it doesn't really make sense unless you still have a CD-burner in your everyday computer, should you happen to download an ISO-file of an old game. At least, this was true until the USBODE came along.
For pure DOS, you do not need a DVD drive, DVDs usually use the UDF file system, which DOS (and Windows 95) cannot read. WInodws 98 have limited supprt for UDF. This also applies to Windows 98SE.
