Node power supply

[MAHRF at de.ibm.com]

Hi everyone,

I'm planning to build a home beowulf of nodes with an Athlon 900 in a mini
tower.
How many watts should the power supply have at least when the only other
parts in the nodes are a NIC, a simple graphics card and maybe a small HDD.
Maybe I'm going to upgrade the nodes to, say, 1,3 or 1,4GHz Athlons in the
future when these are as cheap as the 900MHz models are.
I'm asking because less watts means less heat and it's also cheaper.

Thanks in advance,
Ferdinand