No, that is a computer operating system, iPhones use an iOS which is a completely different language but similar features.
You can run Tiger, Leopard, and Snow Leopard. You cannot run Lion.
Mac OS X 10.6 (Snow Leopard) is able to run both 32 Bit and 64 Bit software seamlessly. (See links below)
The main difference between Mac OS X 10.5 (Leopard) and Mac OS X 10.6 (Snow Leopard) is that Leopard is the last version that will run on the older Power G4 and G5 processors. Snow Leopard and subsequent versions (such as Lion) will only run on Macs with an Intel processor. Consequently much of the differences between Leopard and Snow Leopard were in the inner workings of the system which were rewritten to take advantage of the Intel processors and to strip out the old PPC code resulting in a faster and smaller system.
A MacBook will run Mac OS X 10.6 (Snow Leopard). If the MacBook is currently running 10.5 (Leopard) a copy of Snow Leopard will need to be purchased.
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.
5GB is the space required to install Mac OS X Snow Leopard, but don't expect to be able to run Snow Leopard in that space. You will need at least 15 - 20GB to run Snow Leopard and its apps.
Snow Leopard and Windows 7 are two completely different types of operating system. You cannot use software written for Mac OS X (Snow Leopard) on a computer using Windows 7 (or vice versa). On a Mac you can run both operating systems so you can run Windows 7 instead of Mac OS X or in a separate window on top of Mac OS X (Snow Leopard) using Parallels or Fusion (See links below).
10.5, Snow Leopard is 10.6
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).
An iMac G5 can only run Mac OS X 10.5 (Leopard) it cannot run 10.6 (Snow Leopard) or 10.7 (Lion). So the answer must be 'No'.
It is probably going to come out after OS Snow Leopard
Mac OS X 10.6 (Snow Leopard) and all earlier versions of Mac OS X will support networks.