I've had some luck with mixing RAM, but there's so many different speeds and voltages now.
Not only does it have to be compatible with the board, but also with the other RAM. While I do recommend trying your best to get the same brand and make, a lot of times your motherboard will automatically adjust your timings and voltages if they are slightly different. Even then, it might not run as stable.
With RAM as cheap as it is (practically disposable.), it wouldn't be a bad idea to just buy a brand new kit. Especially when you can get a good 6GB DDR3-1600 set for under $100 US.