Hello, I currently have this memory:

http://www.tigerdirect.com/applicat...

On my ASUS P5B-E Motherboard. They are the 4-4-4-12 CAS Latency. Now, I am planning to upgrade my memory to 4 GB for under 20-40 dollars. This memory:

http://www.tigerdirect.com/applicat...

is a 4-4-4-15 CAS Latency. If I get this one, will it slow down my performance or would it not affect performance? Or would it be better that I go for the same memory I have?

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With the first 3 numbers being the same, you would not be able to perceive the difference.
When you have more than one module, the bios will automatically run all the ram at the slowest ratings of all the modules if applicable.
See your mboard manual - some mboard chipsets cannot run all the modules at their max speed if certain faster modules are used.

The most important thing is to determine whether the ram is definately compatible before you buy it.
See response 5 in this for some info about ram compatibilty, and some places where you can find out what will work in your mboard for sure:
http://www.computing.net/hardware/w...
Correction to that:
Mushkin www.mushkin.com

Once you find the ID strings for the modules that are comptable, you can get them from anywhere that sells modules with the same ID string.