Depends on what you're looking to do. For a stock Core 2 Duo system, you'll need 533Mhz DDR2 (266Mhz FSB 1:1 with RAM). If you're looking to overclock, you'll need RAM which runs at whatever FSB you're looking for, in which case more expensive memory is necessary (though a lot of value RAM will clock to around 800Mhz with a little extra voltage and relaxed timings). From what i've seen, spending extra for RAM that will run higher than 1:1 (ie. DDR2-800Mhz with a C2D @ stock 266Mhz FSB) will give you a slight (generally negligible) performance increase, but isn't generally worth the extra money you spend (someone correct me if i'm wrong on this).