I have a lian li Pc-61. It has 2 intake fans at the bottom on the front, a exhaust at the back toward the top under the power supply, and a blowhole on the top. The powersupply also blows air out, but it doesnt affect it much due to the power of my other fans. They are all 80mm. But my question is, if i were to add other fans to it, where would be the best place to put them? I was thinking adding a intake to the side, but i dont know if that would mess up the air flow. Also i thought about adding another exhaust fan under my one on the back, but i dont know if that is a good idea, as i hear it is better to keep positive pressure inside the case. Do you guys have any suggestions? Thanks a lot for any comments.

A side panel intake might help keep you cpu or gpu cooler(depending on where you put it). It isn't too hard, but If you have to make the hole, you might have to repaint the case unless you're very careful.

I'd vote go for the side panel fan.

And I'd keep the GPU cooler, usually the GPU is warmer, and there's plenty of solutions to keep your CPU cool, much more than GPUs....