Northwood P4s tend to have idle temps in the low to mid 30s and load temps around 40C or so. But it also depends on the ambient room temperature the computer is in. If the room is hot, the computer temps will be hot. If the room is air conditioned, the computer temps will be cooler.
The main thing is that your computer runs stable. If you're getting lockups, freezes, reboots or in the case of the P4 actual slowdowns then the temps may be too high and you'll have to do something about it.
I think as long as you have at least 1 case fan installed on the back of the case to remove heat, you'll be okay.
But really, don't obsess about temps unless you really have a problem due to high temps.
Cricket