PC Overheat Problem. How do I start fan and identify northbridge?

Hello. This is my first post. Thanks in advance for any help and assistance!

Ok here is my problem:-

I bought this second hand custom pc and noticed the temps were quite high on my GPU. I managed to get the heat down from about 100 degrees load with 100% fans to 82 degrees load (3DMark06 running at 1080p with all settings to max and full precision on for over an hour) with fans at 85% (adjusted with rivatuner). I managed this by removing my back panel and reseating the heatsink and applying fresh thermal paste in my GPU. And also most importantly moving the GPU down to the lower PCIe slot as my corsair psu is bottom loaded and blows cool air directly onto the GPU. According to speedfan my system is at a steady 42 degrees.

My four cores run at about 50-55 degrees at load so I am happy with these. However I checked the bios to confirm but noticed that my northbridge temp is at 57 degrees idle and southbridge is at 55 degrees idle. Northbridge rose to 68 degrees running Prime95 torture test (third one which overloads ram) after about 15 mins).

Sorry for long winded description but anyway here is the questions...

Firstly my fan on the top right of my case turns on when windows is loading but upon reaching desktop switches off. So I know it runs, but how do i get it to run continuously as even under full stress and high temps it didn't switch on so I don't think its sensor related either.

Also where is the northbridge as I have read that screwing the screws in hard can lower temps. Are there any other tips that can lower temps, preferably without re-applying thermal paste etc as I am a novice and was uncomfortable doing it to my GPU.

Thank you.

http://i731.photobucket.com/albums [...] MG1391.jpg

http://i731.photobucket.com/albums [...] MG1381.jpg

Sorted pics now. Top pic shows the fan (on right, top) and where it is connected to the mobo).

1) PSU will be sucking in air
2) bottom PCIe is probably only 4 lanes so your GPU is doing less. What motherboard is it?
3) Northbridge is the small copper coloured heatsink in your first picture.