A few days ago i heard about Overclocking, i've understood that it makes your CPU etc. run faster. But that this might overheat the chip and reduce its lifetime.

Other than this i don't anything about it, i've heard that it can be dangerous if you don't know what your doing, like me.
I would be nice if someone could explain the basics, how to do and what NOT to do.
I think my system is cool, but i don't know, since i don't know how to check its temperatur...

My system:
Dell Dimension 4500 - 2002 mod.
Intel Pentium4 - 2,4Ghz
Intel 845c Chipset
512MB RAM
40GB HD
nVidia GeForce MX 420 - 64MB

Overclocking is indeed dangerous, and unless you take massive risks usually yields only minimal results.

You basically change settings in the hardware to override things like clockspeeds and temperature constraints.
These settings are made in the factory to assure the product works well and doesn't get damaged by overheating or overvoltage.
Manufacturers of course build in safety margins in their products. Overclocking reduces or completely removes those safety margins.
Some hardware suppliers selling ready made CDs unscrupulously buy lower spec hardware and overclock it, then sell it for the price of the faster (more expensive) hardware it now appears to be to the uninformed.

My advise: don't do it...
At best you get a small performance increase for the price of a system that breaks down more quickly and crashes more frequently, reducing the lifespan of your PC.
At worst you break expensive things like the CPU and videocard while trying (and remember that any overclocking will void your warranty!).

Plus, chances are Dell has not seen fit to include any kind of overclocking option in the motherboard BIOS, so you can probably rule out the CPU and RAM.

Even so, I run a stable P4 2.4C at 3Ghz.

Overclocking can yield some nice performance increases, but only do it if you:
1) Need the extra speed badly.
2) Have a cpu/motherboard that overclock nicely, and if it's a dell I can assure you your motherboard is NOT a stable overclocker ;P

Overclocking is a fun and often addictive way of learning about how your computer hardware works and pushing it's components to their limits. You do a lot of research about what the different parts of your pc do and their interaction with other components and then perform tests to find just how far you can push these components, finally finding a place where you have raised your performance to a level where the pc is stable and not in danger of cooking itself.
Unfortunately, the quality of your components will dictate whether you can overclock at all. Most overclockers use hardware that has been designed with enthusiasts in mind, motherboards with vast bios options and quality parts, ram that will reach high speeds, aftermarket cooling, and quality power supplies. This usually means that you usually have to build your own system with overclocking in mind.
Most manufacturers of prebuilt systems use parts that will do the job they are needed for and little else. This is for cost and profit reasons and holds little for the overclocking enthusiast to tinker with. Basically, these machines are at their limits at stock, although exceptions are always present in the world in everything.
The first step I would take would be to find out what motherboard you have and whether there are any modified bios' out there that take advantage of any special features it may have (such as FSB adjustment and RAM speed/divider).
Then, if you CAN overclock, do a lot of reading to find out just what you will be doing and why, what results it will give, and what effects it will have on the system.
I get good results from my overclocking and the amount of knowledge I've learnt has been overwhelming. It's a good way to learn about pc hardware.
Just don't forget my warning. It may become addictive.

P.S. One of the best sites for overclocking is www.xtremesystems.org where the big boys play.

well yeah it surely is fun and you get to push your comp to the limits. but hey pls be reminded of the consequences well enjoy!

Basically if you have the money to invest, and are willing to invest, in a new cpu, go for it. I havent done it myself, but im sure it would be quite a learning experience. besides might give yourself an excuse to get a better cpu :p just kidding

Consequences.. yes.. consequences..
I haven't blown my cpu yet (touch wood) but when I was testing my maximum RAM speed I forgot to lower my cpu multiplier down. I was alright until I got to 225MHz HTT and then windows looked funny as it would (not) load.
I rebooted and reset my bios and found I had corrupted my windows boot.
Sure was "fun".

I just did my first OC this weekend, I thought it was quite fun, b/c it's a challenge to push your system as far as it will go, and check it to make sure it's still stable, without messing anything up. I did get an aftermarket cpu cooler with overclocking in mind, but my mobo isn't that great for overclocking, and my cheapo ram did probably hold me back a little, but with the new HS and fan I wasn't too worried about messing my chip up.

little and good you can overclock your dell but not through the bios like most people do... there is a nifty program out there that lets people who have oem locked boards overclock them through windows. Obviously this doesnt work as well as the others but it basically bumps up your fsb which increases your cpu speed and your ram speed. if it is unstable and windows locks up, simply turn off your computer and turn it back on and it resets itself to factory defaults so you can try again with a lower setting. The program is called cpufsb. You should check it out.