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.
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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.