Well, you shouldn't open the case, because, although this makes any part directly cooled by a fan, a whole heck of a lot cooler, it also makes any part that relies on indirect airflow a whole heck of a lot hotter, because they receive no cooling at all (eg, your northbridge if you don't have a fan on it, your RAM, esp if they don't have heat spreaders...) Your normal case design is to take air from the front, pull it past most components, and suck it up through the PSU and rear exhaust fans... Perhaps you don't have a fan in the front grill of your PC, or if you do, it's disconnected / broken? My brother's compy has a P4 HT, and the fans were all screaming high after 5 minutes, because his front grill fan became disconnected accidentally.
I'd make sure you had at least one fan in the front grill, pulling air in, and at least one fan in the rear, pushing air out, in addition to the PSU's built-in fans. A fan in the side is a plus, as is a tube from the side to the top of the CPU cooling stack. (Cut a measured hole in it, run a small length of tube to just on top of the CPU fan, that way it's pulling cold air directly from outside, instead of from the heated interior... this also makes the interior cooler.) I'd also recommend you get an active HDD cooler for each of your hard disks; they're pretty cheap. (Don't get an expensive one, I've been using mine for a few years now with no problems.) Also, grab some heat spreaders for your memory, those are cheap too. All this should cost you ~$50, but they can be used on your next PC as well.