You rarely if ever hear about a computer shutting itself off because the motherboard temperature is too high. That's because if the motherboard temp is too high, it's usually an indicator that something else (like the CPU) is too high, and that's what's causing the machine to shut down.
This honestly does not sound like a heat problem if those readings are accurate. Temperatures in the 40s and low 50s are perfectly normal and shouldn't be causing any problems.
However, I've heard people say that the Asus PC Probe is not that accurate. If it's off by 10 degrees on the low side, for example, then you could be in the 60-degree range for your CPU, and a lot of AMD processors shut themselves down in that area. Does the computer actually WORK when you have the table fan pointed at it? If so, I'd go back to heat.
If you haven't given the whole thing a good thorough blowing-out to get the dust out, that's the first thing I'd do. You may also want to reapply the thermal paste on the CPU, which can get dried out and brittle over time. The other obvious thing to do is add more fans and/or a better CPU fan - stock CPU coolers are often junk.
However, Newegg has REALLY jacked up the price -- as in, almost doubled it -- since becoming aware that people like it. Now it's actually more expensive than a lot of other good options like the Zalman 9500, which I personally prefer.
Before spending that kind of money, though, I would just shell out $10 or so for another case fan. A rear exhaust fan would probably do wonders to improve your air circulation, and pretty much EVERY case I've heard of has a place for one. Since it works with a table fan blowing at it, it sounds like poor air circulation inside the case is the problem, unless the CPU fan itself is going dead. Have you checked the RPMs on it, and does it appear to be speeding up and down properly? Those things do wear out after a few years sometimes.
edit: the Zalman 9500 actually only looks like it's for Socket 775 (duh) which is the last machine I put one on. So you won't be able to use it on your machine unless you can maybe get a mounting bracket. For socket AM2, which is what you have, maybe the Zalman 9700 is a better option. If you do get a Zalman cooler, make sure to read the notes about the clearance you need in your case -- a lot of their heatsinks are HUGE.
well.. i have just replaced the old chasis with new one which have front and rear fans..!! now the motherboard temp is 36"c and my system works fine..
kudos to capt-taco..
will meet again in new thread..!