you will need to buy XP Pro 64bit to see and be able to access all of your 4GB of ram, and your Video Card's ram is not accessible as system memory, do try and learn before you start tinkering with hardware.
He cant run a 64 bit OS on the CPU/Mobo because IIRC, no 64 bit socket 478 CPUs were made. -- Microsoft gives you Windows; Linux gives you the whole house.
