Siddharth Maurya :
ok..ok..I know that I need x64 OS....and it is a desktop PC mobo intel DH55TC(I already mentioned about my empty slots @Barty1884)....
I meant I already have 2x2 GB RAM
I wanna upgrade to 8 GB
So I have 2 options
1) 2x2 GB + 4x1 GB
2)2x2 GB + 2x2 GB
which one should I choose?(I don't wanna remove my old ram)
Sorry, I missed the part about free slots.
Your motherboard supports CP3-10600, maximum 1333MHz
4slots, for a total of 16GB max as per
http://www.newegg.com/Product/Product.aspx?Item=N82E16813121394
Some RAM simply does not 'play nice' with others. There's no universal answer to get either 2x2GB or 4x1GB. Generally either should work but sometimes they simply will not.
Hence, it's recommended....to achieve your 8GB total, you remove your existing RAM and purchase a 2x4GB kit - kit's sold together are tested for compatibility with each other.
Pricewise, a 2x2GB kit is going to be around $30-40, anything higher than 1333MHz will be throttled down, and you have a chance they may not be compatible with your existing RAM.
On the other hand, a 2x4GB kit, which are tested to be compatible with each other and will achieve your 8GB RAM using fewer slots for future expanding (although you'd need to buy the exact same 2x4GB pack.....but you might still find compatibility issues there too!). A 2x4GB kit runs around $40-50
(this G.Skill kit is $40.99).
You're looking at the same pricepoint (or within a few $), for pretty much guaranteed compatibility and allowing future expansion.