Which should i choose: 800 mhz Ram or an 1066 one?

Well if your motherboard supports only 800MHz, then any 1066MHz you put in there will be bumped down to 800MHz, so you won't benefit more from getting a 1066 than an 800. Though if you do upgrade the motherboard to something better and want to reuse your RAM then maybe the 1066 is the better choice, but otherwise, whichever is the cheapest is probably best. Also make sure you're buying for the right DDR. I'm guessing your motherboard uses DDR2, so don't get a DDR3 RAM or it won't work.