RAM speed reduced when I added more RAM to my system why?.?

I just upgraded the RAM in my computer. I originally had 4gigs of corsair xm3s ddr3 memory it was at 1333mhz and had timings of 9-9-9-24. I recently installed 4 more gigs of the same memory so now I have 8 gigs. the new memory has the same speed and the same timings (its also corsair xms3). but in my bios settings the memory speed has been reduced to 800mhz and my timings are now 6-6-6-15. why is this? The slower speed didn't slow down my computer at all I still get the same frame rates in all my games. But what I did notice is that my game play is MUCH smoother. I have windows 7 64 bit. My system is AMD Phenom 2 965 oced to 3.6ghz, GTX570, 8 gigs (4x2gigs) od corsair ddr3 1333mhz memory, Gigabyte 890gpa-ud3h revision 2.0. corsair HX 750 watt PSU and two 500 gig WD caviar blue hdd. The reason why I needed more memory was because in some games my system was running out of memory and the game would crash or shut down. No blue screens. I would get a message that would tell me to shut down the game because system memory was low