|You can vary the amount of memory that is used by there kernel and programmes but your numbers are all wrong. There is no finite number and it dependant for how much RAM is installed as to what actually gets used for what.|
In a 32 bit version of Windows, although you may have 4 Gbs installed, only a little over 3 gbs is available for use. The remainder is used for memory mapped i/o and other system uses that used on every computer, regardless of how much RAM is installed.
The way windows uses memory is that it will attempt to use all available memory. Unused memory is wasted. There are three kinds of memory as far as Windows is concerned.
Allocated and used,
Allocated but unused.
Unallocated and Unused.
Allocated and used is the kernel and running applications.
Allocated and unused is memory that has been used but its contents are kept in memory in case it is needed again and it doesn't have to be loaded from the hard disk. This is what is referred to as system cache. This mainly consists of DLL files and other system file that are constantly coming in and out of use.
This is probably the most misunderstood part of Windows memory management as it is what third party memory managers clear to to free up giving more free memory but can actually slow the system down. It was useful in Windows 98 because the memory management there was flawed. You could end up with orphaned blocks of memory that never got freed or used again until the system was re-booted. In Windows XP it is much improved.
Unallocated and Unused is free memory which Windows attempts to keep to a minimum.
Memory management in Windows is a complicated thing. Unless you understand what is going on it is best to leave things at their defaults because you can get yourself into a right mess and make matters worse.