Most new PSU's come with a 20 + 4 pin plug for the MOB connection. If your MOB is 20 pin, then just use the 20pin plug, and disregard the 4 pin as it is a separate plug. You will have no problems. In fact I did exactly the same thing two weeks ago.
A PSU stated as suitable for a Pentium4 will fill your requirements - however, quality rather than quantity will pay off in the wattage department, although have something in reserve is useful with so many graphics cards having high demands. Some info: