Case Cooling

Hi. I'm running an Athlon 1.2 (not o/c'ed). If I close the case and run a full load, the CPU temp can reach about 70C, usually crashing the system. In the meantime, I've decided to leave the side of the case open, and set my huge 18" room fan on its side, blowing straight into the case. That of course improves airflow tremendously, such that the CPU temp never goes much over 50C, even while running a full load.

My question is, what kind of setup would I need to achieve this same cooling with the case closed, using intake and outtake fans? Also, where should the intake/outtake fans be attached within the case for the best results?

I'm trying to decide if it's actually worth doing something like that, or whether I should just buy a smaller room fan and just set it on the floor, blowing directly into my case, as my large room fan is doing now.

Thanks,
xdarque

Oh, sorry. I have a no-name mid-tower ATX case that I bought about 3 years ago for about $30 at a show, when I assembled my first system, and didn't know the diff between a good case and a bad case. I don't have any case fans in it at the moment. That's what I'm trying to decide: whether it would be worth it to get some, or whether it would just be better (both cost-wise and cooling-performance-wise) to leave the side open and have a desk fan blowing into it.

As for CPU fan/heatsink, I have a $10 "Dynatron Fan/Heatsink DC1206BM-E". Not sure how decent it is. It's the standard fan/heatsink that tcwo.com ships with their OEM Athlons. One of their tech guys, who seems pretty knowledgeable, says he uses it himself in his system, and that it's a much better fan/heatsink than even the one AMD ships with their retail Athlons, despite the low price... (??)