I have a u8668d motherboard with a 1.8 ghz celeron processor. The MB calls for DDR SDRAM memory. PC2100 to be specific. My question is does the memory sticks have to match for this? Could I have a stick of 1 ghz and a stick of 512 in the othe slot? The system seems to lag in some games. especially when there is a game played that has good complex graphics. Also could I use pc 2700 or higher RAM. I was told I could but couldn't get the higher speeds.