Installed 10.5 on MacBook 1.5GB RAM and only 60GB HD with 11.9GB free, upgrading from 10.4.10. It ultimately worked. [BTW, it only required about 6.5GB additional space, not the 11+GB it claimed.] But initially the big reboot to 10.5 did NOT bring up all functionality. E.g., Finder didn't even show my HD (I was pretty worried). Two successive additional reboots brought everything into order for two Users on this machine. Like annealing. All seems OK now with Firefox, Tbird, iLife, iWork, Office 2004 and 3rd parties like MaBaSoft World Clock and Mark/Space Missing Sync for Blackberry apparently working fine. So it may take some patience and holding back panic on the first few boots. [BTW2, I ordered it off Apple website on Wed Ground (free) delivery and it delivered by FedEx Friday morning the 26th. Incredible.] Good luck out there.