there are no case fans fitted, so by the time I added in those
Ummm... if there are no case fans fitted, how are you going to install any? Cut a hole in the case? Perhaps you mean CPU fans?
The cooling will be provided by the PSU fan extracting warm air from the inside of the case, with ambient air entering via louvres in the front, and possibly side & rear also. All the internal components that require it will probably use passive cooling - i.e. heatsinks, although a 1 GHz CPU may well have a cooling fan in addition to a heatsink.
Very often the inside of the case has baffles to duct the air over components that particularly require cooling - it's there for a reason!
SD