How do I know system is over heating or not?

Hi I just finished building a computer last weekend and I've had it running for the whole week testing stuff out, setting it up the way I want it and what not.

My computer has never crashed due to overheating but I would like to know when the limit is, because I don't want to cross it.

My system is :

MSI K9N2 platinum SLI
PNY 9800GTX+
Phenom x4 9650 @ 2.3GHz (with stock fan)
2 x 2 GEIL 800MHz RAM
Hec X power 650W SLI

Case :
Antec 900
2 x 120mm front fans (on high)
1 x 120mm side fan (on high)
1 x 120mm rear fan (on high)
1 x 200mm top fan (on high)


my CPU temp on idle is 31 - 34 (NEVER HIGHER!!)
CPU on load for 3 hours playing COD4 on max everything : 49 - 51

GPU fan on idle : 41 - 44
GPU on load for 3 hours on COD4 max everything at 1366 x 720 reso : 56 - 61



all the degrees are on Celsius.


and i'm sorry if this topic is long, but if i add another 9800GTX+ will it pretty much destroy all air flow to my 9800GTX+ that i have now? or will the side fan be good enough to supply fresh air to the GPU / GPU's?



Thank you,
Joey