Dual channel question !

Because you have mismatched amounts of memory, the system will not utilize dual-channel.

Is the difference significant? Depends on what you do. Dual-channel memory can be twice as fast as single-channel memory. If you play lots of games or do any intensive computations, dual-channel will probably be worth it, and a 2x2GB DDR2 kit isn't that expensive. Although, depending, again, on how intensive your games and computing are, you may be able to get by with just the 2x1GB running in dual-channel mode.

If all you do is browse the web and type up word documents, its not worth buying more memory, then its just a decision of do you want 4GB (single-channel) or 2GB (dual-channel).

thank u for ur reply ,anyway i use my system for gaming and im running win7 64bit
so should i use only the 2x1 !?
another question if i get 4x1gb does this support dual channel ?!

The more reason for you to upgrade the mem to maximize your OS plus added performance too. You could get another 2x1gb dimm together with your existing 2x1gb to get 4gb assuming that your mobo supports 4 dimm slots. Just be sure to put the dimms in partners to the correct dimm slots for you to run in dual channel.

The thing your seem to be not understanding, you must install memory in identical pairs to run dual channel. 1 stick, no. 3 sticks, no. 1 stick of 1 gig and 1 stick of 2 gig, no. IDENTICAL SIZE STICKS IN PAIRS ONLY for dual channel operation.
I would do as above poster mentioned, simply buy another 2 gig stick to match the one you have, and install the 2 sticks in the correct slots on your board for dual channel. (check your manual for the correct 2 slots) Toss the other memory you have, or give it to someone.

thnx guys ,,but if i want to install another 2 gig stick should it be the same model
or just the same speeed ?! i mean 800mhz and different model
(i have 2gig cl5 and the only one available to buy is cl6)will this work (mainly for overcloking )?!
and installing another 2 gig stick without removing the 2x1 would give me 6gig
in dual channel right ?!!!and what is the best setup for overclocking ?
sry for asking a lot of questions !  

No and no and no.

For Dual channel, you want 2 Identical sticks in the proper place. Don't mix memory, don't put an uneven number of sticks, don't put different latency sticks. 2 Identical memory sticks!!!

That means you have to buy 2 identical memory sticks and only use those 2. Sell what you have now.

^Uh, yeah pretty much everything he just said. You are still not listening.
Dual channel mode requires 2 IDENTICAL sticks of memory per bank. 2 Identical sticks per bank. NOTHING ELSE WILL WORK IN DUAL CHANNEL MODE.
While some will say that you can successfully mix different brands, or latencies, you are asking for problems. In fact, you are beggin for problems. What you need is 2X2 sticks of paired memory, for 4 gig. Buy a 2x2 gig kit, get rid of what you have if you cannot find an identical matching stick.

Now before someone jumps on me, there were some boards way back when DDR was first being used, that say they will run a single, or 3 sticks in dual channel mode. They however did require a certain type of double sided memory stick to do this. Just covering my bases because there is an exception to every rule, and around here there seem to be people who thrive on jumping in and driving home every tiny insignificant point that really means nothing to the majority of us.
(like the guy who keeps jumping in to assert a 32 bit OS that can use more than 4 gig of memory, my god I am so tired of that one)

Thank u very much guys , u helped me a lot! but i read this article : http://www.intel.com/support/motherboards/desktop/sb/cs...
and :
and this pic explains dual channel using thre sticks!!!!!!!!!

does this apply only to intel mobos ?!
anyway im gonna go with 2x2 ,hope i can find an identical stick ! i dont think i can sell what i have !!!
thnx ....

You should really look at your mobo's manual for information on placing dimms to run in dual channel. Some motherboards (MSI, i think) uses the other method.