I heard once that it's best to leave the side on to help with airflow... However I ran one of my comps without the side on without a problem(though it died after being stored in my closet
Anyway, I'd leave the side on and purchse some fans for the back and front if you don't have any. Those would probably help more than anything. And it looks like something right behind your computer. Give it more space to breath!
Yeah you can fit a couple of fans at the back, That'll help it, Thing with leaving the side open it'll after a while collect a lot of dust, By the looks your case looks air flow friendly so just bust some fans in there and you'll be a happy chap
Not only could dust get in it but other little things such as small peices of paper or you might spill something in it. I would say that is not a good idea.
What I read a while back was an open case caused hot spots on the mobo because it cut the air circulation. *shrugs* I could see this happeneing to an extent, but I wouldn't think it would be too bad. Over all, it probably is the best idea to leave the case on and just use the fans as they were designed. Keeps it more quiet too
Acording to A+ any open areas on case make for bad air flow.. even missing slot covers.. If you want better cooling simply buy a heat sink that fits your CPU such as a zalman 770 or xp-90 and place a fan over it thats around the 80mm range. its pretty simply just do a google search on those and go from their..