The advantage of water-cooling compared to air-cooling are that your system is cooler, but it also produces less noise.
Off course, the reason why I choose for water-cooling is that you can overclock higher then with air-cooling

Before I started all this I informed about what would be the best solution for my system, which is:
A voltmodded Epox 8KHA+
An Enermax Power Supply
An Athlon XP 2000+


The waterblock:
I chose for the newest version of the Danger Den Maze II which can be bought @ Loveno.

You can find the tubing and ties at your local home-improvement warehouse.
Make sure you can´t bend the tube, otherwise you can get some problems with the water flow and the flow of your pump will reduce. The size of the tubing should be 3/8

The waterpump:

I used a Mebner B501, that produces 800l/hour, and that´s more then enough.
You can find water pumps in your local garden centre, there are enough types and possibilities.

Radiator:

If you´re not planning to overclock, this is not really necessary, but if you don´t use a radiator, make sure your reservoir is big enough (>5 litre), so the water can cool enough. A radiator doesn´t have to be expensive, you can find it in garages with car-parts, or you can buy one for almost nothing from a friend, this one comes from an old Porsche :

Copper and aluminium aren´t best friends. The material of your block and radiator should be the same. The reason for this is that the water transports ions, and this way, the two materials could react with each other. To solve this problem, you can use an additive. Good solutions are cool-liquid or Waterwetter, or you can do it like me, with a combination of both.
I used 4 Sunon 60mm Fans and 1 80mm Papst NGM for my cooling.

If you really want to buy one, I would take the Black Ice Xtreme.