If possible, you want to avoid blowing air out of the bottom of the case- that's the coolest air you have. If you mean by "blowholes" empty pci slot areas, or some type of grillwork, you would do better covering those up, and opening up the top unused cd-rom bay cover. They usually just pop off. You might need to look around some, and take off the from cpver of the case- it varies with the design.

The idea is to blow in air at the bottom, and force it out the top, along with the heat it has picked up. Case air cooling is more important than most people think, as it directly affects the cpu temps, too.

I even go to the trouble of removing front metal grills that are usually blocking the front bottom air intake of the case fan- by whatever means necessary- files, dremel tools, metal cutters, etc. If you do this, remove all boards and componets first, and be sure and completely clean out any filings or metal pieces. They will kill the motherboard, or anything else they touch with a short out. However, it is really worth the trouble, if you have a heat buildup problem.

sorry- another typo- I mean "front cover of the case"

Hmmm... my system crashed the for first time now with opened case. It really seems that the CPU fan isn't doing a good job.
But great, I think, we have pinpointed the problem. Now it's time to solve it, by whatever means...

Most stock heatsink/fan combos that come with a computer aren't generally up to the job of cooling Athlon cpus. I would try to do something about case airflow, and get a better heatsink/fan. Be sure the heatsink you select will fit your board, that is, clear all the other parts, like capacitors. Also consider the power connector of any more powerful fan- the board connector, or PS connector. It will use slightly more watts, and with a questionable 230w PS, that's a factor.
Good luck!

wrc1944