Underclocking memory to better OC processor

My mobo's multiplier is maxed at 10x. So all I can do is up the frequency to increase my speed. I'm trying to hit 2.2GHz from my stock 2.0GHz on stock cooling/parts so I need to hit a multiplier of 220MHz.

I was wondering, how do I know how much I should underclock my memory so that I get a better OC? It's rated at DDR2 800 5-5-5-12 at 1.9V.

I know I can underclock my memory to 667 speeds but how do I know what timings to use?

Thanks in advance.