I have an old Dell Inspiron 6000, which I have found has 2 memory slots, a capacity of 2gb of memory, and uses 200 pin DDR2 RAM, 533 MHz. If I were to use memory that is a HIGHER MHz rating, such as 667 MHz or 800 MHz? There is almost no inventory of 533 MHz but lots of 667 and 800. Also, is there any benefit to using one 2gb stick vs two 1gb sticks?