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Any 64-bit version of Windows XP, 7 or 8 manages your ram for you and uses as much as you have. You don't need to do anything.
32-bit versions limit the use to 4 GByte only and there is nothing you can do about it. There are some complex kludges to try to make it use more than 4 under some circumstances but they are mostly a useless waste of time.
64bit version of Windows will automatically give all the programs as much RAM as they need. Windows even has technology to use your free hard disk space as RAM replacement, so that it can give more than 8GB of RAM if programs need more than what you have. When exceeding 8GB your computer would slow down because hard disk is so much slower than RAM, but all the programs would still get what they need on Windows's default settings.
If you have 32bit version of Windows, then the limit is 4GB. You should use 64bit version of Windows.
Well, the only caveat being:
You need 64-bit version of Windows - that was explained well enough.
But it's also per application. A 32-bit application can still only access 2G (3G under certain circumstances), even if you have 64-bit Windows, and even if you had 100G of RAM available.
That being said, you can run as many of those applications as you want up to your RAM limit, and Windows will indeed automatically allocate your RAM just fine, it's just that you can't adjust how much RAM your programs are getting - they take as much as the developers tell them too, and they have a 32/64-bit limit as well.