I have an Intel Core 2 Duo with 2.0 ghz. I heard someone on the forum mention "overclocking" it to 3.0 ghz (ghz may not be right, correct me if I'm wrong on what those are called). I also have a video card that was overclocked out of the box. How wouid I go about overclocking my CPU to 3.0 ghz?

I think advice on overclocking should start with an explanation of what that really means. It's not like it's some form of black magic.
A digital clock is simply a voltage source that goes from low voltage to high voltage (for example, 0V to 5V) and back several (million?) times per second. Since it does so near-instantly, this creates rising edges (transitions from 0 to 5 volts) and falling edges (transitions from 5 to 0).
Your processor (and every other digital device) needs an incoming digital clock to operate, since it does something whenever it receives an edge. The clock is supplied to it by your motherboard. Usually it's "CPU host clock" routed through a multiplier. Since you can usually control both via BIOS you can control the clock frequency that ultimately goes to your CPU. Higher clock means more edges per second, which means more operation per second (which means more energy, which generates more heat, so you can't set it arbitrarily high).
I don't know whether this particular post helps or not, but ultimately it simpler to be aware of some basic digital design concepts than to start with tweaking BIOS having no idea what it really does. (Which is how I started, and which is how most of the people start.)

Which is what gives rise to all the YouTube videos of people overclocking their machines insane amounts, whilst pouring liquid nitrogen directly onto the CPU. Lots of fun, but in the long run, not healthy for the hardware either ;-)

Quite frankly, the first questions should be: Do you need the power... and if you do, isn't there another cheap way to do it? Overclocking can (depending on a lot of factors), dramatically decrease the lifespan of your CPU/GPU and requires you do carefully watch the thermometer for a while to make sure that your cooling can handle the added heat. If you can get similar power, for example by buying a bit more memory or a newer, but still pretty cheap gfx card, then that's the way to go. Overclocking only makes sense if the the overclocked components are very cheap in comparison to standard components with similar power.

I wouldn't necessarily agree with that. While overclocking will lower the lifespan of your chip, you're talking on the order of reducing its life expectancy from something like 15 years to 10 or 12 years. And seriously, how many people don't upgrade their systems before that amount of time has passed?
As well, no matter what chip you go with, if it can be overclocked, you're going to gain performance making it a better deal than buying a slightly higher default clocked, more expensive chip.