UPGRADING RAM

Currently have 2-1gb PC2-6400 667mhz and I wanna max out my memory to 2-2gb PC2-6400 800mhz. Will the difference in Mhz matter, and will i even be able to use a mhz thats higher than what came with my computer??

What CPU and motherboard you have will determine whether you will see any benefit from getting DDR2-800 RAM over DDR2-667 RAM. Some CPUs and chipsets won't run DDR2 at over 667 MHz, so if you get DDR2-800, it will just clock it down to DDR2-667. If your CPU and chipset support running at DDR2-800 speeds, you may see some performance benefit in going from DDR2-667 to DDR2-800 in addition to the performance benefit in going from 2 GB to 4 GB of RAM. The amount of benefit varies by your particular CPU- if you were running a CPU that really expects DDR2-800 like a dual-core or quad-core AMD CPU, then you will get a noticeable improvement. If you are running a lower-end or older Intel CPU with a slower (533/800 MHz) FSB, then you probably won't notice much of an improvement as the FSB already had as much memory bandwidth as it could handle with the DDR2-667.

I could give you a better idea of whether or not the upgrade to DDR2-800 would yield performance gains over DDR2-667 if you posted the CPU and motherboard/chipset you are using. However, I wouldn't worry too much about it as simply increasing the quantity of RAM will most likely be the biggest boost for your system.

As others have suggested yes it probably will work. You can also try checking on the motherboard manufacturers website to be sure. You may need to manually change the FSB:RAM ratio manually in the BIOS, as I had to, to get it running at the right speed.