Settings advice, fried my last game card mid 2011 iMac

[123pblack]

Hi Guys, 

RE: IMac 27" Mid 2011 
3.4 Ghz i7
32 Memory Ram 
512MB Game card, The card I fried was a 1GB, The repair guy added 512MB ( I didn't ask why because it just out of warranty)
I Don't know a great deal about computers internals.

Using ProTools 10HD on a 10.7.5 Lion Partition & 
Final Cut on a 10.11.6 El Capitan.

I think I fried my last game card because the corner the computer was in didn't have enough ventilation, I wanna make sure this doesn't happen again & moved the Mac near a window which is a lot cooler.

Also I've been told that the most expensive part you don't wanna overheat of an iMac is the Mother /Logic Board?.
Which fan is nearest it?

Also what settings should I use & sensor should I pick to view along the top. I'm only allowed to choose one from the list.
(Some guy told me don't let your computer run over 60 C , he didn't say which sensor value tho)

There is also control options 
'Auto' or 'Constant value' 

If I click  'constant value' sensor then I have an option 
 'Sensor - based value' At the standard settings the fans seem to run a little bit too fast!

Should I use 'Constant RPM value' and if so what is a good value to use or 'sensor - based value' what is the best settings to generally use?

If I click  sensor – based value
ODD Fan:
Standard is set at ambient 20 C to maximum 30 C 
(What does this maximum really mean?) 

HDD & CPU: the same 20-30c

Going with the settings seems to be running fast enough! 

Thanks for any help in advance!

B W
Paul