I have a 2gb and 4gb ram of 1600mhz of samsung .can i add a 8gb corsair vengence 1600 mhz ram ?

Tradesman1

Legenda in Aeternum
You can try, there's no guarantees when mixing DRAM. Briefly dual channel is when you have a like (same) amount of DRAM in each channel and the MC (memory controller) treats it as a single 128bit device rather than 2 separate 64 bit devices (as each stick is an individual 64bit device). In your case here with a 4 and a 2GB stick, if they are in slots 1-3 or 2-4 then they will be running in whats called Flex mode 2GB of DRAM from each channel is running dual channel and the odd 2 GB from the 4GB stick is in single channel mode.

More on Flex in my article FAQs, item 7, here:

http://www.tomshardware.co.uk/ddr3-dram-faq,review-33220.html

If you add an 8GB and it will play nice, would put the sticks from the CPU as 4GB - 2GB - 8GB - empty. In Flex this will put 12GB in dual and the odd 2GB in single
 
It will most likely work. It's not like the old days when you had to get exactly the same brand or there was a good chance you would have problems. Things have changed sense then and while there is a chance still that the RAM won't play nice together it's rather slim. I myself have mixed many different brands of DDR3 RAM together and I've never had a problem.