So right now everything is connected to the internet via wifi from the airport express. I was wondering if I would see faster speeds on my iMac if I connected directly via ethernet?
Depending on your imac, airport base station and internet speed it could, but proabley won't. It will only make a real difference (when talking about traffic between your computer and the internet) if your internet connection speed is over the speed of your wifi (which will most likely be 54mbp/s or 300mbp/s).
However it will dramatically increase internal network speeds, for example, transfering a backup of one of my xbox cames from the storage drive on my server (around 8gb of data) takes about a hour over 54mbp/s wifi, 20 - 30mins over 100mbp/s eth and 5-10 mins over 1gbp/s eth