I've got two 4G ram sticks in my PC right now. Thinking of buying one Corsair Vengeance 8G stick to add to the group. Will this cause any problems? Don't know what I have in there right now or the speed. Would I be wasting my money ... aka ... do I need to take out my other memory to install this or can I just plug it in?
You can't just chuck a single dimm in, this will kill your memory bandwidth by half by disabling dual channel. You will need to purchase 2 additional dimms of identical make in order to minimize compatibility problems, or just straight buy a 16gb dual channel kit for what might as well be zero dollars.
You need to know your motherboard type. Like @korwin said, you risk a performance decrease if the dual channel is switched off. What you want to do is see if your motherboard supports dual, triple or even quadruple channel for the memory. It's probably dual, in that case you want to buy two identical sticks. However it sounds you have 3 or maybe even four slots. So be sure to check, if you have four slots with two times dual channel you can also buy two sticks to have a total of four. If you have triple channel you want 3 identical sticks, if four channel you want four. If you feel like reading, here is an explanation.
Match you RAM. If you're buying dual channel you need two sticks of identical RAM. If you're buying triple channel you need three sticks of identical RAM. If you want to fill all four of your dual channel RAM slots, then you need to match them all in specification (latency, speed, voltage).