CPU-Z shows 668 mhz frequency on ddr3 1600 pc how to increase speed?

Your system is using the default setting, You need to go into bios and tell it to use 1600. Normally all you need to do is to tell it to use XMP. On my MB when selecting XMP, I then select Profile 1 and for memory I just left everyting for memory on Auto.

Added. When you do this it is always a good idea to run memtest 86 from a bootable CD (min 2 hrs.) BEFORE loading system to verify no memory errors.

1600 MHz 9-9-9-24-2T @ 1.5v {F3-12800CL9D-4GBRL} http://gskill.com/products.php?index=222

668.7 X 2 = 1337 MHz

HOWEVER, if you have a 2.66 GHz CPU then the process is different to go above 1333 MHz; you'll need to enable DRAM OC Profile.

I have a i5 760 @ 2.8ghz so i'm about 2.66

Close {Horseshoes, Handgrenades, Nuclear Weapons}; it's not 2.66 GHz.

Use AI OC Tuner {Auto}
DRAM OC Prifile {Auto}
DRAM Frequency {DDR3-1600}

DRAM Timing Control / enter
// set timings 9-9-9-24

2nd Information
DRAM Timing Mode {2N} -> 2T

DRAM Voltage {1.5v}

K ill try this what do you mean about horseshoes and hand grenade

It's a saying "Closeness only counts in Horseshoes and Hand-grenades" I added "Nuclear Weapons" to be funny - so when you said -> "so i'm about 2.66" "About" doesn't count it's not exactly 2.66GHz. If you read your manual then you'll understand.

i'm sorry   I re read horshoes and hand grenades 5 times figuring my mind was being stupid, till i copied and pasted and it was really what u typed.

i did read my manual though, my dislexia really fucks with numbers. i'll re read it.

The good news is that I did read the manual, I knew about the 2.66GHz BIOS setting differences which is why I asked. The post above takes into account your CPU and they are the correct settings.

Doesn't i5 760 run at 2.8 not 2.66?

Yes, the i5 760 does run at 2.8GHz - http://ark.intel.com/Product.aspx?id=48496

I am and was 100% clear. If you had a 2.66 or slower CPU the AI OC Tuner settings are different. The settings I posted are for CPUs > 2.66 like yours.

The following {repeat} is correct:

Use AI OC Tuner {Auto}
DRAM OC Prifile {Auto}
DRAM Frequency {DDR3-1600}

DRAM Timing Control / enter
// set timings 9-9-9-24

2nd Information
DRAM Timing Mode {2N} -> 2T

DRAM Voltage {1.5v}

Ya I'm not saying you were wrong by any means I'm having a hard time finding the tight BIOS settings

Download the PDF Manual - http://usa.asus.com/product.aspx?P_ID=T2FxW2fXGZQgSn2V

refer:
3-15 {p 75 pdf}, BIOS - overview and layout
3-16 {p 76 pdf}, AI OC Tuner // also note the 'grid for CPU speeds'
3-17 {p 77 pdf}, DRAM Frequency
3-18 {p 78 pdf}, DRAM Timing Control
3-19 {p 79 pdf}, DRAM Timing Mode
3-20 {p 79 pdf}, DRAM Voltage

when i went into bios it said that the ram was already running at 1600 but cup-z still says 668.8 mhz.

Thanks i figured out how to check it through this post. so is this a bios error? or is the memory maxing out at this frequency.

I bout 2 pairs of dual channel ram is it automatically in dual channel or is that a setting.

Two pairs suggest 4 sticks of RAM, are all 4 the F3-12800CL9D-4GBRL??

Also, only look to the right-most column; in this example JEDEC #4:

was i looking at wrong screen? i was looking at the "Memory" tab, not the "SPD"

My SPD says XMP 1600 then below 800 mhz

DDR {Double Data Rate} 800 X 2 = 1600; you're good!

With XMP all of the timings 'should be' set automatically, and the Manual settings above are/were unnecessary.