NO, ITS NOT WORTH UPGRADING your friend's computer. With a 120mhz proc, he probably has EDO ram, <24x CDROM, 28k modem, <2gig hdd & an AT case. At this point he's better off buying an entirely new system!! All he can keep from the old rig is the mouse, keyboard, floppy, & monitor.
But if he doesn't have the money, keep the CDROM & hard drive too, and look into integrated motherboards -either way you need a new case.