Can you add a different brand and speed RAM to your computer?

I built my PC about a year ago and I want to install more RAM. I have 2 sticks of 2gb OCZ ram. I wanted to boost my system to 8gb but my heatsink for my CPU actually covers one RAM port on my motherboard (yea sucks right?). So, I wanted to get one 4gb stick to add in the open port. Rather than buy two sticks of new ram to replace my old ram, can I just buy one new stick from a different brand (maybe a corsair 4gb stick) and add it to my system of two OCZ 2gb sticks and have it function properly? I do not know if you can mix brands and speeds of ram. They are both compatible with my motherboard, I just do not know if they can work together without problems. Thanks in advance.