You have to purchase the new 64bit Windows on disk and install it on your computer or the computer you are looking to buy. Double the RAM amount for the 64bit is the standard rule. If you had 256M RAM for the 32bit, then you would use 512M RAM for the 64bit.
You won't get much of a performance enhancement with Windows 64bit on a home install; sticking with the 32bit and finding a web refrence that allows you more control over such features as SMP and virtualization may be a better choice.
Gateway is currently offering the FX540B for $699. This seems like a good deal on a solid machine. My question is I would like to use for RAM. How easy is it to updrade to windows 64 bit? Is it just buying a download and then adding more RAM? I appreciate everyones help.