Using 2 rams with different companies

Different companies work yes, but the speeds have to be the same. If you have DDR2 800MHz, then you need to get Corsair RAM that is DDR2 800MHz. But DDR3 is cheaper, so its probably not worth even buying DDR2 these days.

Does your mobo support DDR3?

Ram is sold in kits for a reason.
Ram from the same vendor and part number can be made up of differing manufacturing components over time.
Some motherboards can be very sensitive to this.
Although, I think the problem has lessened with the newer Intel chipsets. Still,
it is safer to get what you need in one kit.

You do realize that you will need a 64 bit OS to access more than 4gb, don't you?

Ram is relatively cheap these days. buy a single kit with what you need.

You want documented ram compatibility. If you should ever have a problem, you want supported ram.
Otherwise, you risk a finger pointing battle between the ram and motherboard support sites, claiming "not my problem".
One place to check is your motherboards web site.
Look for the ram QVL list. It lists all of the ram kits that have been tested with that particular motherboard.
Sometimes the QVL list is not updated after the motherboard is released.
For more current info, go to a ram vendor's web site and access their ram selection configurator.
Enter your motherboard, and you will get a list of compatible ram kits.
While today's motherboards are more tolerant of different ram, it makes sense to buy ram that is known to work and is supported.

No my motherboard doesn't support ddr3 , so i think i'm going to buy a 4gb corsair ddr2 ram , so does the corsair produce 4gb ram in a single stick .

as said the ram should really be bought as a kit, but you can get away with mixing and matching if you do it properly and make sure the speed is the same across the jedec and the timings are the same for at least the primary 4. if you can match them up exactly then even better. i recommend you go to the ram manufacturer and either find there ram stick spec sheets or email em and ask which ram they have that matches your current ram...

Hey i have decide to buy a ram kit of 4gb but i'm confused between whether i should buy kingston 2x2gb or corsair 2x2gb which one is better and which one has more waaranty , my budget is 40 $ .