I'm looking to upgrade my laptop memory from 2Gb (2 x 1gb) to 4Gb but am not sure what memory speed I need. Some websites say that I need 667mhz ddr2 for my Acer 5920G however some say that I can use 800mhz ddr2. Will there be any problems using the 800mhz (which is also cheaper) if the mobo doesn't properly support it?