Portal 2 requires an up to date version of Snow Leopard. The requirements are:
Mac OS X 10.6 (Snow Leopard) will not work on a Mac with a G4 processor as it requires an Intel processor based Mac. A G4 (867 MHz and faster) can run Mac OS X 10.5 (Leopard).
Mac OS Leopard
Currently, it's $129 dollars (Mac OS X Leopard). Once Snow Leopard is available (August 28), it will be a $30 dollar upgrade from Mac OS X Leopard.
Mac OS X 10.6 (Snow Leopard) and all earlier versions of Mac OS X will support networks.
Yes, Mac OS X 10.5 Leopard ships with Java support.
To upgrade from Mac OS X 10.4 (Tiger) to 10.5 (Leopard) you will need to buy a boxed version of Mac OS X 10.5. If you have a Mac with an Intel processor you can go straight to Mac OS X 10.6 (Snow Leopard) which is a cheaper upgrade option.
Mac OS X is the operating system used by G5 Macs. The latest version of Mac OS X that will work on a G5 is Mac OS X 10.5 (also known as Leopard).
Mac OS X 10.5 is known as Leopard. The last available version of Leopard was 10.5.8.
10.5, Snow Leopard is 10.6
Mac OS X 10.4 (Tiger) can be updated to Mac OS X 10.4.11 for free. Purchased upgrades then are Mac OS X 10.5 (Leopard) or Mac OS X 10.6 (Snow Leopard). Snow Leopard only works on Macs with an Intel processor. Older Macs with a G4 or G5 processor can only go up to Leopard.
Although the appearance of Mac OS X 10.5 (Leopard) and Mac OS X 10.6 (Snow Leopard) is similar there are many differences in the workings of the two systems. Most notably Snow Leopard requires a Mac with an Intel processor whereas Leopard will run on earlier Macs with G4 or G5 Power PC processor as well as the newer Intel based Macs.
Mac OS X 10.4 was called Tiger and Mac OS X 10.5 was called Leopard. The 10.5 (Leopard) version is better than the 10.4 (Tiger) version. They would not have sold many copies of Leopard if it was worse than Tiger.