The difference will be virtually imperceptible. The best thing you might do if you don't want the hassle of sending them back, is downloading something like CPUZ and seeing on the SPD tab what the lowest latency you can run the sticks @ 667mhz and then making sure that is what they are running at.

If you can lower the latency settings it'll probably make up for not having the higher clocks.

For example my sticks are rated CAS 8 @ 1600mhz but if you run them slower like 1333 you can run CAS 7. The performance difference is virtually zero at either setting TBH, higher clocks is only slightly better in very very few circumstances

I didnt even know all this about RAM until after i bought it. I already checked with cpu z and under DRAM frequency i'm running at 664.9Mhz. So that's about 1330 but not sure about the rest. will post pics here maybe they'll help. https://db.tt/MLTJ8y2X and https://db.tt/dEYYqaa6 Maybe someone can enlighten me on the figures on there. Good to know though that the performance difference will not be enormous with higher clocked ram

The slowest slot 1 is a 1333mhz module, the other one is a 1600mhz module. It's double data rate so it's double the clockspeed quoted there for actual speed.

You obviously run at the speed of the lowest stick, but clearly you can see the RAM in slot 2 says it can do CAS 9 - 9 - 9 - 24 -33 @ 685mhz same as the other one @ 666. You have them at 665 or thereabouts.

So make sure the timings above are correctly set in bios for maximum performance @ 665mhz. The difference in performance between 1333 CAS 9 and 1600 CAS 11 are miniscule.

Bios can't really be set. Its a dell laptop. Studio xps 1645 to be exact. So it just defaults to running at the fastest speed of the slowest stick. In this case the 1333mhz module. I have been stressing about it as i wanted this laptop as old as it is to run as fast as possible. Now atleast I can put my mind at rest if the difference will be as minimal as you state. Thanks spartan2. Glad to see that you are still around after all these years here on pro offering help where you can

i got a ?.. so we can run 2 diffrent brand ram stick togather? or it wont work? coz i gave im plang to upgrade my ram to more 8gb an thinkb of get gskull stick..n my frnd said ul need same brand of ram to upgrade

you can run two different brands but its NOT advised. You are best off running the same brand and same clock speed sticks. I could only upgrade mine one at a time hence the issue and only got concerned when i didnt see a massive jump from 6gb to 8gb like i was expecting. I didnt know any better at the time but based on my experience now go same brand same frequency/speed. The main indicator however if you go different brands and pc rejects the setup is the pc will not boot.

tnx kid... looks like il need to sell my ram n get a pair of 2 8gb gskull ram...curently have transend 1333mhz 4x2(puke)

I wouldn't even use the same brand and model RAM together unless they came in a kit. The chips used for the memory can change, even if they still are at the same speeds. You want chips all from the same batch, so a kit.

get corsair vengeance. U get lifetime warranty . replacement in case of problems