It is a fairly hot British summer day, my room temp is 29C. I am running my system (Athlon (Thunderbird) 1200 ) with one side panel off a mid tower case.
I have a Silverado on the cpu, plus two case fans (front inlet and rear exhaust) which each move 46cfm.
My system (motherboard) temp was 32C and cpu temp was 47C (100% cpu load).
As an experiment I disconnected the power to both case fans, within fifteen minutes my temperatures had increased to: system 39C and cpu 53C.
Whilst my system is considerably quieter without the case fans running I conclude that unassisted convection cooling is not very effective.