With overclocking, you have two basic things you can adjust: the multiplier and the Front Side Bus (FSB). The Pentium 4 has a locked multiplier, meaning, you can't adjust it, so overclocking is relegated to the FSB.
The Pentium 4's (and newer Pentiums) have a quad-pumped FSB. This is very important, or you'll furiously be trying to OC something you can't. The FSB's base frequency is multiplied by 4 for the real FSB, so a 400MHz FSB is 4x100MHz. Now the CPU's true speed is determined by the CPU multiplier and the base FSB. So 28 x 100MHz=2.8GHz.
Some BIOSes have a special name for it, but you're looking for a 'Frequency and Voltage Controls' menu. What you'll be looking for may be called FSB, but you may also see 'External Clock Frequency'.
This is what you want to adjust. Try a 5MHz bump. This might not seem like a whole lot, but, if you've got a high CPU multiplier, this will net a nice little OC.
You're not done yet. Get ahold of [google]Prime 95[/google] and [google]Memtest[/google]. Memtest can be put on a bootable CD or floppy disk, and this will hammer your memory. Prime 95 will hammer your CPU in Windows if you run the tourture test. The goal is to have no errors in either program. You'll want to run Memtest for 3-8 hours to beat it over the head, and Prime 95 for a full 24 hours. Now, you can use your system with P95 running, and it should work fine.
Okay, let's say you get some errors in either program. If DRAM voltage adjustments are available, you might bump this number up by 0.1V and try again if you're running into memory problems. For the time being, I'd leave the memory timings set to Auto. That's an indepth discussion on it's own.
If your CPU fails, try bumping it up by the smallest amount available (typically 0.05V) and try again. For the P4, DO NOT go over 1.7V or risk fatally damaging your CPU.
Temps:
If you see your CPU temp hitting 60*C or higher under load, or worse--idle, you'll want to look into some better cooling.
