Ok I'll give it my best shot.
Your clock speed is the product of the FSB and the multiplier. Take your Barton for example... it comes stock with a FSB of 166mhz and a multiplier of 11, for a clock speed of 1830mhz (11x166).
The Front Side Bus is the frequency that your RAM, AGP, and all of your PCI slots run on, so if you bump up the FSB you increase the clock speed's of all of these devices (unless you have an NForce2 board with agp/pci lock, but we will get to this when you tell us your system specs). If you increase the multiplier, you are only increasing the CPU frequency.
So in order to change the speed you must go into the CMOS setup (tap del at POST). The clock speed is almost guarunteed to be located in the first section listed, but if it isn't there just keep looking. You will want to increase the FSB in small increments (maybe 5mhz for the first jump up). A 5mhz FSB increase will increase your CPU clock speed by 55mhz (5mhz higher FSB multiplied by 11). The one thing you need to watch most is temperatures. Do not let the CPU get above 50C to be safe, but the lower the better (I am currently idle at 29C). You can test stability by using a program to "stress test" the system. Prime95 is probably the most well-known stress tester, and what I personally use.
Now you have to be careful if you are FSB overclocking to not let your PCI devices get too far out of spec. The PCI frequency should be kept as close to 33mhz as possible at all times. You can determine your PCI frequency by taking the FSB and multiplying it by the divider. The most common dividers are 1:4, 1:5, and 1:6. For example, your 2500+ runs at 166mhz FSB, so the motherboard will automatically set the PCI divider to 1:5, giving the PCI devices a 33mhz frequency. But if you increase the FSB, the PCI frequency goes up with it. The AGP frequency is another thing to watch out for. It is found by doubling the PCI frequency, and as you have probably guessed needs to be kept as close to 66mhz as possible. Everytime you increase the FSB 5mhz, the PCI will go up 1mhz and the AGP will go up 2mhz. Get it?
*Ignore the above paragraph if you have a PCI/AGP lock on your motherboard*
Another thing you need to watch out for is yoru RAM frequency. The RAM frequency is equal to the FSB (unless you have a motherboard which allows you to set it independently, but I don't recommend this as it is proven to give lesser performance than when the RAM is "in sync" with the FSB). Again, once you give your system specs we will be able to tell you what your RAM frequency should be. It is alright to overclock your RAM, just make sure you are constantly stress testing. Your RAM or CPU will cause a system lockup on a stress test long before you get the frequency high enough to the point where it would damage your hardware, providing you overclock in small steps.
If your system locks up or reboots while stress testing you have two options. You can either back the frequency down a bit until it is perfectly stable (Prime95 can run for 24 hours straight), or you can increase the voltage.
IMPORTANT: Increasing voltage causes your system to run hotter, and if increased too much can kill your system. The stock voltage for a Barton is 1.65v. Depending on your cooling setup (it better be good if you want a high overclock), you are safe taking your vcore (cpu's voltage) to around 1.725v-1.75v. Just check your temps constantly if you up the vcore. You can also increase RAM voltage to make your RAM more stable. This is done in VERY small increments, so be careful or you WILL fry your RAM (I wouldn't increase it more than one notch unless you have some good cooling, and only increase it if absolutely necessary).
Cooling is the single most important factor of overclocking. High speed fans, copper heatsinks, and Arctic Silver thermal compound are your best friends if you are serious about getting a high speed. If you just want a moderate overclock then you can get away with a lesser cooling system (but what fun is that?)
Just do me a favor and keep an eye on your temperatures at all times....
*I am not responsible for any damage done to you or your hardware. Overclocking isn't that risky if you are smart about it, but stuff does get broken. Overclock at your own risk*
Hope this helped, and good luck!!
.. and dont forget to write posts not novels
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