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.
Hoot