Question about DDR3 memory?

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 (stock speeds i didn't touch anything). I recently installed 4 more gigs of the same memory (same specs as my other two memory sticks 1333mhz 9-9-9-24) so now I have 8 gigs (4x2gb of corsair xms3 DDR3). 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) 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. Can i just go back into bios and adjust the speed back to 1333mhz and put the timings back to what they used to be? Or is the new memory timings going to make up for the reduced memory speed?