I think I get what you're saying but it doesn't work that way.
A dual boot system means you're running one OS or the other. You're not running both at the same time. Each OS has it's own limitations.
To run both OSes at the same time, you would need to run one OS as a virtual machine (I think this is where you're thinking RAM drive). While you could run an 8GB 64-bit linux OS, you would still need to assign X amount of RAM to an XP virtual machine and if the VM was a 32-bit OS, assigning more than 4GB of RAM would be wasted.
-Wolf sends