You could also consider not putting a side fan on. Unless you have a particularly hot GPU, you may actually be disrupting the front-back and/or up-down airflow in the case.
Experimentation is the name of the game here. Get HWMonitor and run your PC to see what your internal temps are like. Then install the side fan. See how the temps are affected. Flip the side fan to the opposite sense and go again...takes time, but you'll work it out from that.
i definitely have a hot gpu, ever since my previous side fan stopped working it's been getting up to 190 deg F (88 deg C) -- with my face i have a 300mm outbound fan on the top, a 120mm outbound fan on the back (near the cpu), an inbound fan on the front at the bottom (probably not doing much since that's just where the extra psu cables are, it's below the hdd and gpu) and this side fan will be right next to the GPU, so i'm thinking out ...
i guess the last post is probably the best overall idea though, try both and see what gets me better temps!