At what point do I have to start to worry about my temperatures?

I am short on space so I have to put my PC in a desk which restricts airflow.
There is 1 80 mm fan on the side of the case (to draw in air) and one on the back (to expel air),
but there are no fans on the front of the case to draw in air.

My system temp is 39 deg C and my CPU is 54 deg C.

I even got the CPU up to 64 degrees when I closed the desk doors.

When I take the PC out of the desk I have these readings after 1 hour idle
Sys:30, CPU:47

With my new case fan after 1 hour idle the temps were
Sys:22, CPU:43




My system is
NF7-S rev2
AMD Athalon 2000+ (Palomino)
Stock AMD HSF
ABIT Siluro GeForce FX 5200
512 MB (DDR SDRAM)
Maxtor 40 GB Hard Drive (84320D4)