Had 8GB of spare memory so popped it into an old Intel Core 2 Duo E4600 and installed Windows 7 x64 bit as the processor was x64 compatible.
Checked windows system and noticed it had 8GB (3.25GB usable). Strange, so I upgraded the bios to the latest version and checked that it was seeing 8GB, which it was.
I noticed under msconfig.exe (just type it into search and you will find it, or usually under C:\Windows\System32)
Click "Boot" Tab
Click "Advanced options"
"Maximum Memory" was ticked - so I un-ticked it and rebooted
And now I have the full 8GB of ram available.