Can I insert 2gb ram and 1gb ram in a mother-board
Cab I insert 2 GB ram and 1 GB ram in the slots provided in the mother-board?
Knowing the motherboard that you have would be very helpful but lets say that you are using a current motherboard like the ASRock Z77 Extreme4. With that board you would be able to put a stick of DDR 3 1600 2GB in it and another stick of DDR 3 1GB in it and it would work but it would only be in flex mode and give you single channel performance.
So based on the board in question it is possible to put two sticks of memory on it with different sizes and have it work.
yes, typically you can mix and match ram sizes in a motherboard. We would have to know your exact mobo to know what capacities/voltages/etc. your mobo will take, but generally speaking, if you match the specs of your current stick of ram with what you purchase new then everything should work fine.
If you get ram with slightly different settings, just know that all memory will run with the slowest speed and timings between both sticks. So if you have 1 stick that is DDR2 800, and annother stick that is DDR2 667 then both sticks will run at the slower 667 speed.
Keep in mind that running mixed memory does not allow the use of Duel DDR, which can give a nice speed boost if your motherboard supports it, so I would generally suggest that you only use ram in identical pairs for best performance. But if it is an older system, or performance is not a large concern, then don't worry about it.
Lastly, and this is very important, you cannot mix ram of different generations. If your system uses ram that is DDR2 then it will not work with DDR1 or DDR3.
If you have specialty ram, meaning it uses lower than normal voltage, or is buffered, or uses ECC error correction, then you will have to match the type of memory used exactly in order for the computer to work. These types of memory are very rare, but it is something to look out for (especially the voltage).
Oh! also, keep in mind that motherboards have a maximum amount of memory that they support. If it is an older system (like a Pentium 4 or something) then the system may only support a total of 2GB of memory. Anything that has a Core2Duo or newer CPU should be able to fit 4GB (or possibly more) without a problem.
Again, this is based upon the specific motherboard used, so we would have to know the exact make and model in order to give a definitive answer on that.
My other build I have 2x2gb sticks and 2x1gb sticks making a total of 6gb, running in dual channel as well. 2 different brands, 1 set is corsair while the other in PNY, works great and Overclocks well together. But normally you can mismatch sets or size of ram. But I would keep it running in dual channel if you can.