This is a relatively simple topic, so I'm going to start off at the top. Hope I'm not starting too low.
This is all assuming that your motherboard supports changing the multiplier setting. If it doesn't, there is no point in unlocking your Athlon.
CPU speed is made up of two components: bus speed and frequency multiplier. Your 1800 runs at 1.53GHz, or a 133Mhz bus speed * 11.5x multiplier. The Athlon does not really run at a 266MHz bus. Well, it does in the same manner that DDR Ram runs at 266Mhz. It doesn't by the pure definition of clocked timing, but effectively it does.
To overclock, to say 1600MHz, you have two choices: Set the multiplier up to 12x, or leave the multiplier alone and increase the bus speed. A 139MHz bus speed * that 11.5x multiplier would also yield 1600MHz.
Increasing the multiplier only adds increased stress to the CPU, but does not increase the speed of other periphals and only yields performance boosts on CPU-bound tasks.
Increasing the bus speed will yield performance boosts across the board, but causes increased stress on RAM, video card, and all other PCI devices as well as the CPU.
Increasing the bus speed is preferable if all of your components can handle the stress. Some cannot. The only real way to know for sure is to test.
Hope this helps.