|I believe you're confusing RAM frequency (which is the true speed) with the advertised DDR rating (which is double the frequency). DDR2-800 actually runs at 400MHz frequency...sometimes the speed is listed as 400MHz, sometimes as 800MHz. I'm fairly sure what you have is DDR2-800, not DDR2-400.|
"Please tell me can I use the two RAM sticks"
Of course, but ideally they should be a matched pair so that you can enable dual channel mode on your motherboard.
To confuse you even more, your Q8400 runs at 1333MHz FSB which is actually 333MHz frequency. So if you have no plans to overclock, all you really need is RAM that runs at 333MHz frequency...that would be DDR2-667. You can obviously run DDR2-800 with a 1333MHz FSB CPU but the configuration is not optimal. What you should do is either underclock your DDR2-800 RAM to DDR2-667 speed, or overclock your CPU to 1600MHz FSB to match your DDR2-800 RAM speed.