Laptop memory has gotten much more afforable recently so that price is probably correct. I am purchasing an extra 4GB stick of RAM for my G74 and it's only $30! The things you need to match with RAM are the MHz and the PC#, for instance mine is PC10600 1333MHz. So as long as I match those specs, I can get any size I can afford. There is a limit to how much RAM the motherboard supports but from what I've read the G73 supports 16GB just like the G74.
I am not a RAM pro but I have upgraded my RAM in the past, someone else might have more info about the efficiency side of it. For instance I have read RAM works in pairs but I dunno how true that is. I have used different #GB ram in the same computer with no issues. As far as what order to put them in I haven't heard anything about that.
If you can afford it, I would replace the 2GB sticks with 4x4GB. That way you have a ton of RAM and don't have to worry about the order since they're all the same
😉I believe upgrading the CPU is possible but it will void your warranty.