I've got an old Dell desktop to use as a backup/spare. It's running desperately slow on win 7 with just 1 gig of ram. I'm going to put some more in. It currently has 4 X 256 ddr2 400mhz. can I use higher speed ddr2 ram, 667/800 mhz etc and it'll default to the 400mhz as per the the system spec? I'm probably looking to put in 2 X 1 gig.