I do not have any experience with liquid cooling in computers.
So who better to ask then the best in the buisness (you guys).

My system:

Abit BP6 Dual cel500@566
G-Force 256 DDR
IBM Deskstar 46Gb
235W Power Supply

As you all probably know, the BP6 has very little space around the CPU's and tampering there is very difficult.
I'm thinking of making my own water cooler.
The design is my own. A massive copper block with three straight drilled holes. I don't know if a just a single large block is such a great idea, thats why I'm asking you guys.
The demensions of the block would be 50x140x25, and the tank is also an issue.
How large should it be?1L? 2L?

[Eriksson]

If you can fit one waterblock to the BP6 there is nothing wrong with that. However if you are going to cross drill this thing you will need quite large holes, or many small, to make good cooling. The actual size of your tank is not important. You might not need one if you got an inline pump. The tank is just used for submergible pumps and perhaps as a temp buffer, or to compensate for leaks. Since leaks are not desired that purpose is irrelevant.

You will need a good cooler, heater cores from cars are good to experiment with. In general you need less fan with bigger cooler, or to be accurate you can run your 'big' fan slower. One 120mm 12V fan running at 7V might be enough if you got a good cooler. However some ppl seem to need more fans than others, and computers sounding like 747 in takeoff are currently in use. However the main advantage with watercooling is how quiet it can be, if properly installed.

Just remember: a fan is no replacement for common sense!! Good Luck