Memory Question

[Homey D. Clown]

I Know this is probably a newb question but I 've searched this site and googled it but I still haven't come up with a clear answer. So i'm running a AMD6000+ Dual Core at stock, no overclocking and no oc in mind, 2 Gigs of Corsair Value Select 667, yeah I know not the greatest but it was free!, In The motherboars Bios there is an option under one of the memory settings lists DDR2 400, DDR2 667, DDR2 800. To run in sync should I choose 400, 667, or 800?