I just bought a new Dell Dimension E521 and want to replace its case fan with a quiet fan. It is a 120mm fan that blows from the front of the case, over the CPU heat sink, and ehausts to the rear.

The fan uses a 5-pin connector to plug in to the mother board. But it only has 4 wires to the fan. The new fan I have in mind only has 3 wires.

Can someone please answer my questions below:

1. What does the 4th wire do? Will I get an alarm if I only use 3 wires to the new fan?
2. Where can I find a 5-pin to 3/4-pin conversion kit?

Thanks.

David