Take a look at the "plate" that your main board screws into. The piece of metal that holds the whole mainboard to the rest of the case. If it's one solid piece you may want to do what I did. Put some holes in it so that air can circulate around the back side of your board. Not too many, you want to maintain good support for your board. But get some room for air to get around back there. Trapped heat loves to hang out back there. I rigged up a small thin fan that blows air onto the board form the back. I cut out a small hole with my dremel tool and then just used a small bit of super glue and my hot glue gun to get a nice looking seal around the fan so It fit nice and snug. Now my board is a happy camper. If you've already got holes back there which I'v noticed that most cases have them on there now, try rigging up something like I just explained to move that stagnant heat out of the way.
Start with adequate and well thought out ventilation. In a mid-tower, if your HS fan blows into the HS, mount a Sanyo-Denki 92x32mm 55cfm fan in the rear blowing in and a Panaflo 80x25 47cfm on the front blowing out. If your HS fan sucks air off of the HS, reverse the setup. Whichever fan port you wind up mounting the 92x32 fan on will need to be enlarged with a nibbler and hand drill. A small extra effort, but well worth the trouble. That is IMHO the best return for effort in mid-tower case cooling, without going radical.
im going to go into watercooling soon, but at the moment i have got a thermo engine in a slocket, so cutting away the solid mobo plate probabley wont do much, there is no room to add active cooling to my bx chip becasue of the slocket and thermo engine, there is a delta on the thermo engine a120mm ys-tech blowing in and a 80mm delta and psu blowing out