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I have 2 x 1GB on 800Mhz CL4 memory in my PC. I'm planning to upgrade to 6GB by adding a pair 2GB modules.

Is it ok if I buy CL5 modules or do I have to buy CL4?

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What operating system are you using? You should be aware that you need to run a 64-Bit system to support more than 3,5GB of RAM. – xor_eq Jul 15 '10 at 12:54
I plan to upgrade to windows 7 64bit – user42966 Jul 15 '10 at 12:57
i have just done the same and pc is running ok , 2x1GB CL4 Sticks and added 2x2GB CL5 Sticks , just trying to research these settings i think the sticks have different stats ex 5-5-5-18 i think the newer ones are less so not sure if i have to alter settings in bios or not :-) – user378754 Oct 12 '14 at 8:09
Not true on two counts. 1) a 32 bit OS can use way more memory with PEA. 2) The 3.5GB is not 3.5GB but 4GB minus PCI[e] address space. It is often rounded to 3.5GB but it can be more and it can be way less. 3.5 is a nice average though. – Hennes Oct 12 '14 at 13:37
up vote 2 down vote accepted

yes, should be fine, but all will run at CL5.

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