It would be like buying a Corvette and never exceeding 55 mph. It will work just fine, it's just not being pushed. If anything, you can run more aggressive timings.
Adding different RAM speeds can cause instability. I would either put the 800Mhz RAM speed down to 667Mhz if you already have it or buy another 667Mhz kit, because mixing kits can give you BSOD, hangs, and many other issues. Don't do it.
Adding different RAM speeds can cause instability. I would either put the 800Mhz RAM speed down to 667Mhz if you already have it or buy another 667Mhz kit, because mixing kits can give you BSOD, hangs, and many other issues. Don't do it.
So long as you run the ram at the speed of the slowest module you should be fine.