If you have a 32 bit OS and a 64 bit Processor, the processor only use the the 32 bit of his Buses (adressbus, databus , etc) and thats the reason why you only can use 4 GB RAM.
If you were right, Vista and XP could not use half of the RAM in the PC because in all modern PCs the RAM is 64 bit wide, or even 128 bit sometimes, just as the processor data bus. By the way, happy birthday1
