I have searched for some lga775 cooler tests and it seems my results are pretty normal for the stock Intel cooler.
I have also found on Intel's site that the CPU temp should stay below 63,5C, and it's recommended to have system temp. below 39.
I have also found someone saying his motherboard runs at 45C and ASUS replied to him that it's fine with this motherboard.
So I think I'm just within the limits. (except for what Intel recommends for motherboard temps)
Downloaded some system monitoring software and did a few tests.
After starting my PC, running on idle CPU cools down to 40-41C. Under load it goes up to 58-60, which is high, but when I stop it cools down quickly. (CPU fan speed varied between 1070-1170 btw)
Another thing though doesn't do that, the system temp. It goes up from 35-36C to 40-41, and doesn't cool down. (very slowly went down from 41 to 40) So I guess the problem is airflow here.
I have only one 8cm cheap case fan at the back blowing out the air. I guess I should replace it with a 12cm fan and put another 8cm to the front blowing fresh air inside the case. Now that'll be a new fan too as the current one can produce quite a noise as it's old. Probably not working at 100% performance as well.
Just by putting a new PSU in my temps went much cooler compared to the temperatures I saw when my old PSU's fan stopped working, so I guess it'll do the trick.
It's scary to think that I've ran this comp. for a year without that 8cm fan at the back, and another 3 years close to the limit, especially that the 9600GT produced much more heat than the Radeon HD5570 or the 7600GS. I guess I am lucky that I didn't have a major problem, though it might have something to do with the video ram going bad on the 9600GT.