Have a look at the stickies in the overclocking forums, they'll probably be of great help.
I think madshrimps.de have a good A64 overclocking guide - it'd be the same for opterons....Here
it is.. and it's madshrimps.be, not de... Good job I had a look myself, or I'd have sent you on a wild goose chase...
Have fun! :mrgreen: