If I want to upgrade my RAM, should the new RAM be of the same brand and speed? Will it work with a different speed and brand?
(I have a gskill 4gb 1333Mhz ddr3 ram, will it work with cosair 4gb 1333Mhz?)

Help is greatly appreciated!

If I want to upgrade my RAM, should the new RAM be of the same brand and speed? Will it work with a different speed and brand?

There's no guarantee that different Ram sticks ( with different speeds , timings , voltage ) will work together. That's why they sell Ram sets instead of selling single Ram sticks.

If speed, timings and voltage match then things are a lot easier. If not your motherboard will change DIMM settings to use them together. If the motherboard can't do that , then you'll be greeted with a BSOD on boot.

(I have a gskill 4gb 1600Mhz ddr3 ram, will it work with cosair 4gb 1600Mhz?)

Speeds match. Can you also specify their timings and voltage?

From experience, there's a 99% chance that RAM with different brands/speed/latency will work together as long as the voltages are the same.

The remaining 1% is almost always due to a crappy Intel motherboard, the H61 and H67 models being the worst offenders.

Thanks guys, my motherboard is a gigabyte GA-H61M-S2P. Sorry in the topic I mentioned 1600MHz, it should be 1333Mhz.

And where can I buy g skill ram? (I bought earlier from Techzone, and prefer another place instead )