Is it a good idea to add a 2GBx1 to existing 2GBx3 RAM? Motherboard has four DIMM slots total.

[Petermas]

The motherboard has four slots for the RAM sticks. It states them as DIMM_A1, DIMM_B1, DIMM_A2, DIMM_B2 in that order. Currently three of the RAM slots are occupied with F3-10666CL9T-6GBNQ 2GBx3 1.5V DDR3-1333 CL9-9-9-24 and I'd like to add to the first slot a F3-10666CL9S-2GBNS 2GBx1 1.5V DDR3-1333 CL9-9-9-24

The 2GBx1 stick is going into the DIMM_A1 slot and it has S and S instead of T and Q on the description as seen above in bold. What is this difference mean and will it be a problem?
What worries me most is whether I am wasting my time and money adding the single 2GBx1 RAM cause I know the current RAM setup runs good together as 2GBx3. Will the 2GBx1 that goes into DIMM_A1 slot interfere with that? Just to clarify, this now means that all four DIMM slots are occupied with RAM sticks.
The PC is from 2008 and getting slow (used for gaming also) and I thought to add a bit more RAM to help.

I was hoping that the single 2GBx1 stick would run separately from the 2GBx3 sticks such that it took care of background computer stuff so as to free more RAM for the 2BGx3 to dedicate to the gaming part. Could that happen?

I'd appreciate any insight into this matter,

Petermas

 

[Petermas]

The 3 sticks you have are a triple channel memory kit.
If they have the same number of ranks (r1 or r2) as the intended addition it will most likely work with or without a little messing about.So if it does not match, find a match for the rank no.
It is hard to say anymore on the limited info you give.
Basically you currently have an un balanced system which is either running in single channel mode or it is running in asymmetric bank interleave.
Getting 4 sticks running will enable dual channel mode or symmetric bank interleave, so yes it would pick up 4 to 13% performance depending on the age of the system.
Performance gains moving from 6 gig to 8 gig would be around 5 % but again it depends on the rest of the system.
You have a possible 15% more performance on the table.

Hey thanks for the input tea, but I am confused about the "...if it does not match, find a match for the rank number" talk. I gave all the information I could see on the RAM sticks, I mean that is the relevant info right? So anyways, what is this rank number (r1, r2) and where can I find its relevance?

Thanks for your time,

Petermas