I think you mean 1200Mhz for the system bus ? if this is so then it depends on your mother board, if its a single channel version you would need a DDR2/3 capable of running at 1200Mhz for a 1:1 ratio. If you have a mother board that is dual channel ready for RAM all you would need is DDR2/3 600Mhz (600+600=1200) but as always buy the fastest RAM you can afford it will reward you with better performance.
... as always buy the fastest RAM you can afford it will reward you with better performance.
Actually, these days (especially with Intel CPUs), the processor core clock speed pretty much controls the performance, so it's better to spend your money on a faster/better CPU than on faster RAM, as long as your RAM can run at least 1/2 the CPU FSB speed (as JD described above, this would be DDR2-600 for a 1200MHz FSB speed).