It's okay to have mixed ram, but not ideal. It will work.
3 is an odd number. If you're gonna get more ram, it's better to get 6GB (another 3X2GB) so that your triple channel is all the same. Mix up the channels so that you have one Wintec and one other brand in it.
You're gonna have to buy ram with similar specs. Voltage should be the same so that your MB will run them both at the same voltage. If you buy faster ram, it will only run at the slowest speed of all your ram.
You can also buy the 3X4GB kit, but they're pretty spendy too.