In addition to a 64 bit operating system, you will also need the right hardware in order to use 8 GB of memory. Both the CPU and the chipset must support 64 bit computing, and must be able to address at least 8 GB of RAM. All of Intel's chipsets since the P965 meet these criteria, as do all Core 2 based CPUs, including the Pentium Dual Core and Celeron E1x00.
8 gigs of RAM on the Gigabyte GA-X38-DQ6 board
The motherboard has to support "memory remapping". On some boards, this feature can be switched on or off in the BIOS, but it can be hard to figure out where the feature can be found, and even what it is called. In some cases, it is activated together with some of the CPU features. If in doubt, your best bet is checking your motherboard's manual.
Memory remapping on an Asus-X38 board
The BIOS finds 8 GB as well
On the AMD side of the fence, all processors for Sockets F, 939, 940, AM2 and AM2+ offer support for 64 bit computing and thus 64 bit memory addressing right out of the box. The chipset does not play a role here, since the memory controller is integrated directly into the CPU.