There really isn't a "normal" temperature, it depends on each systems case cooling, the ambient air temperature, the type of heatsink, CPU usage and peripherals. (Other components generate heat and warm the CPU's environment)

However, there are abnormally high temps. Over 55*C I would start looking at a cooling solution. In hot weather, if the PC is not in an air conditioned room, consider removing the side cover or using a high volume case fan.

Leaving the system on constantly may cause high temps as the entire metal mass of the PC as a whole will heat up like a big heatsink, only dissipating what heat  it can through radiation or the usual inadequate case fans.

Many people seem to think that stopping and starting some components (particularly monitors) will reduce their lifespan. Well, it's probably going to be time for a new monitor anyway by then in my opinion.

It's my opinion that leaving a PC on constantly is just a waste of power and extra wear and tear on the system, especially if their are any moving parts operating.

Leaving a PC hooked up to the internet is also a virus/attack risk.

In my experience, systems get screwed up after a few hours of operation anyway, and need rebooting, which is probably soley due to operating system and software glitches. RAM fragmentation still seems to occur even with the latest WinXP op sys.

Saying that, there are a lot of PC's out there that run constantly, but I doubt many high end gaming machines would do that successfully.

I run a P4 3.0 chip and it idles and 35-40 C and goes 45-50 under load.

Intel P4 3.0GHz
Gigabyte GA-81K1100
Corsair DDR XMS3200C2PT TwinX (2x512 matched in dual channel)
2x Maxtor DiamondMax+9 (7200rpm)
ATi 9800XT-256Mb
Audigy2 ZS 7.1 THX
Antec TrueBlue 480w
MSI cd/rw + dvd
Coolermaster WaveMaster Case

This is currently stock air-cooled but will go over to water shortly and then be completely rebuilt and upgraded in the Spring.