OK - have just been trying to learn how to borg (borging, ?) and remembered:
... running the DOS client with the "-advmethods -forceasm" switches turned on. ...
It is also worth making sure you have the 3.25 Client (available for download from the 'Beta' page @ Stanford). IIRC the 3.24 client only applies -forceasm to the first work unit: if it runs continually the subsequent work units will not benefit from SSE/3DNow?
(And first attempt at adding to the farm is hardly earth shattering: a P500 that at least has SSE!)