It depends on your motherboard, as some BIOS's have more adjustability than others...
Take a read at the
Technology Guides and Tips as I'm sure there's a good write up from a fellow forum member on everything you need to do...
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My Rigs:
Athlon 64 3400+ (CG) @ 2.4GHz, Asus K8N-E Deluxe, 2 GB OCZ DDR400, BFG nVidia 6800OC 128MB, Seagate 200GB PATA, Seagate 160GB PATA, Dual Boot XP and x64, Antec 350W Mid-Tower
Athlon 64 3400+ (E3) @ 2.4GHz, Asus K8N-E Deluxe, 2 GB OCZ DDR400, eVGA nVidia 6800 128MB, Seagate 320GB SATA, Vista x64, Antec 350W Mid-Tower