I7 920 - 3x4GB to 4x4GB Memory upgrade impact

After you add other RAM in the empty slot then the RAM will run dual channel mode, and it does not impact performances.

http://www.tomshardware.com/reviews/Intel-Core-i7-Nehal...

If this is a computer used for gaming, moving up to 16 gigs won't improve performance.

You will gain zero performance. Just keep the 3.

Thanks everyone for replying.
My purpose for adding one more stick is:
I am using couple of VMs for testing and development.
And I run some Softwares which require much more memory then regular needs.
I never use this for Gaming.
So my memory usage reaches almost 10-10.5 GBs(Saw it in task manager).

@cin19
as u said(through link), dual channel will affect overall only -0.6%, so should I add it?

@Azn Cracker
I don't want to gain performance but just want to confirm that my system don't loose performance.
So will I loose some performance or it will stay the same as cin19 said.

It'll pretty much stay the same in performance. There aren't many workloads that most people make extensive use out of that benefit from memory bandwidth significantly. However, mixing memory kits can cause stability issues and sometimes doesn't even work at all, so it's usually best to get all memory as a single kit or at least duplicates of the same/nearly identical kit. If you're only adding a single module anyway, at least make sure that the specs are identical to your current memory.

The 1% of difference means nothing. Add the RAM, but if you don't buy the RAM yet and like blazorthon said find the same/ or similar speed and timing one. Most of time there should be fine to mix the RAM, but don't rule out they don't work.