Yes, it should work just fine, not recommended mixing the RAM,
but in most cases it works just fine.
It will get down-clocked to the lower speed of your other stick.
Nikorr is right, mixing RAM can cause issues, but most of the time it works just fine. Take note that in addition to different timings and latencies that will cause all of the sticks to downclock to the lowest common denominator, it will not run as efficiently as it will likely disable dual-channel if you only have one stick in the other channel instead of two.
I've ran into this issue before where you have a 2 sticks in dual channel then add 1 more stick in one of the other slots and it disables dual-channel altogether, thus giving you higher capacity, but reduced throughput.
So do you need a greater capacity or better throughput (speed)?
A greater capacity, yes, for sure. I don't know what and how the speed affects the system though... is 400Mhz bad? How big would the difference be if both my sticks were at 800Mhz?
Don't worry, the speed is so fast that u will not be able to see it with your own eyes : )
Its all in milliseconds.
Check this fast RAM, and with slower RAM its similar.
RAM speed comparison - We're looking at a less than 2% difference from the fastest to the slowest.
Your biggest worry is, if it works. If not, than resetting the BIOS is in order.
Can u reset the BIOS? Reset the CMOS memory the right way as is described in the motherboard manual using the clear CMOS link with the power cord unplugged. And when the battery is out, press the power button to discharge all the electricity from the system. How To Clear CMOS @ http://pcsupport.about.com/od/fixtheproblem/tp/clearcmos.htm
Ah, all's looking good then, I'll be buying the RAM! Thank you for the help!