Finally solved my computer problem. It turns out that Kingston PC3200 memory is incompatible with my ASUS A7N8E-Deluxe motherboard.
It took me a month to figure that out. I was convinced that it was a problem with Radeon 9800 Pro.
I had to reset the FSB speed from 200 MHz to 166 MHz, but at least Half-Life 2 and Far Cry are running without a glitch now. I can't really notice any performance difference. So I am pretty happy now.