Are you getting 2.8ghz using a higher Fsb 233x12?
well it's not necesary to underclock your ram to get stability, as far as i know having a high frecuency RAM on a low frecuency fsb doesn't cause any problems. Your mobo is able underclock it for you to match your FSB : DRAM ratio.
So lets asume you have right now a fsb of 233mhz, your mobo can select a 1:1 ratio so your ram is running now at 467mhz (233x2). In this case since your fsb is so low you don't need to change anything to your ram, since it could handle up to a fsb of 400mhz, and that would be for you 400mhz x12 = 4.8Ghz.
so if you want to use a higher ram frecuency, set the ratio to 1:1 in your bios and start pushing your fsb to overclock it.