Which motherboard are you using and what kind of memory setup (3x2gb eg.) ?
Does your BIOS recognize all of the 6 GBs of memory at boot? If not try reseating your memory sticks, or try different slots if you can.
If this is a Windows only problem you might want to check Control Panel -> System and Security -> Administrative Tools -> System Configuration, select the boot tab and advanced options. If the maximum memory is set to 4GB there disable the memory limitation option there.
If you have 'X' amount of memory installed, and either the BIOS, or Windoze is only showing 'Y' amount available, the very first thing to do is: pull your CPU, get a flashlight & magnifying glass, and check your socket for bent or distorted pins:
Nineteen times out of twenty, you will find 'em! Excellent pictorial guide to installation/examination from Intel.
...if the pins are not too badly mangled, they can often be 'teased' back into place, using:
a jeweler's tweezer (most large hobby shops)
a jeweler's screwdriver (any dollar store)
a dental pick/tool (often found at dollar stores or hardware store counters)
a 'pin-vise' with a sewing needle mounted in it (a 'pin-vise' is a little thing the size and shape of a kid's crayon, that has a tiny chuck at one end, much like a drill chuck - they are used to drill ultra-fine holes in small work, and can be found, again, at any large hobby-shop or hardware store...)
...just takes a steady hand, and a bunch of patience