Windows 7 improvements have made the operating system essentially a mandatory upgrade for all PC users whose computers are capable of running the program. Windows 7 64-bit allows higher-end machines to make greater use of their functionality, such as RAM of over 4 GB. While having more RAM, up to 8 GB, can help Windows 7 64-bit run, it isn't necessary for the program.
Windows 7 is going to use whatever RAM is available, be it a maximum of 4 GB or 8 GB. The main difference in having 4 GB or 8 GB is in the percentage of the RAM used by the operating system at any time. Having more RAM to begin with simply means that Windows 7 has more RAM from the start, which requires less use of the hard drive in running programs.
With more RAM available, Windows 7 is able to run programs noticeably faster, but often not to an extent that makes a huge difference. Often, the speed difference between 4 GB and 8 GB of RAM is only within a few percentage points, making the speed boost felt but essentially minor.
The difference in RAM speed mainly depends on the programs being used. Simple programs, such as a word processor or Internet browser with minimal add-ons and tabs, don't demand much from the RAM and therefore display little difference between 4 GB and 8 GB. Using more intense programs, such as image editors, multi-tab browsers with several add-ons and graphically-intense video games, generally benefit from having a greater amount of RAM to keep all the processes running smoothly and quickly.
Similar to running programs that demand high resource use, running several programs at once can push the limits of the system's RAM. With Windows 7 already using a chunk of the system's RAM, that leaves less for the other programs. A boost in RAM leaves more resources for the computer to use with these other programs, which can make multi-tasking, even multi-tasking with intense programs, possible.
Essentially the choice of upgrading to 8 GB from 4 GB for Windows 7 64-bit comes from usage. While a greater amount of RAM will almost always result in some boost in speed, PC users who constantly run multiple, resource-heavy programs simultaneously, including graphic programs and high-end games, make better use of that increase than those who use their computers for more standard, everyday functions.