Thank you for your answer.
I did some more testing today. I turned on my PC in the morning and did some surfing while drinking my coffee (20 min or so). Then I ran 3DMark 06 and about 20 seconds into the first test the sound of "sparks" came back. It was only for a brief moment, but it came back time to time while I ran 3DMark. After I finished the benchmark, the sound never reoccurred.
I then proceeded to reset the BIOS to the defaults as you suggested and reseted it (I have everything on auto anyway, not much to reset). I reran 3DMark as soon as Vista booted up, and the sound was still present.
I sold my Radeon 3850, so I can't try switching the graphics card. I'll see if I can borrow one somewhere during the weekend tho.
I also tried plugging my PC into a different electrical outlet and I've tried switching the graphics card onto the second rail and putting my harddrives/DVDRW onto the first (I basically switched it around). The problem still persists.
It does seem to get worse as the system heats up, that's for sure. I ran 3DMark again after that and the first pass I only had about three occurrence of "sparks" in a really short interval of perhaps a second or less. I immediately reran the test when it finished, to keep things warm(er), and it gradually got worse (more occurrences and longer intervals of "sparks"). Which is basically what I noticed yesterday when playing COD4 - it got worse and worse to the point that It just got to scary to continue playing.
I tried to observe the inners of the PSU from the backside while running 3DMark to see if I can catch anything extraordinary, but I can't really say I saw anything odd.