Beginning with beowulf

[Santiago Muelas]

Hello,
I am a newby to this list and to the beowulf world.
As it seems to me that the level of this list is quite high, perhaps I should find another for "beginners". If so, any suggestion on the possible list will be very welcome.
My question is a simple (and I guess a sempiternal) one. I have just run my first parallel program in a "beowulf". I have used up to five processor in an intranet in this School of Enginnering. Everything run o.k. with just one "small" unconvenient: the best result was obtained using three processors, but almost the same as with two. Using the five was a total disaster. 
The reason seems clear. One computer is a dual one. The network is a totally standard ethernet 100M.
Now the question:
What would be your advice about network cards (giga-ethernet seems clear but would they be fast enough?), and switches. My plan is not tu use more than ten computers in this first period of approach to beowulf. 
 Could you give me some indications about the characteristic of the cards and switches if needed. Would it be possible to do it without a switch and making the conections directly between computers? (I have no idea, but perhaps using a couple of cards per node...). I am using Athlon XP 2200 and 512 DDR in each node, and fortran90+MPI as software.
Is this correct?
Many thanks in advance and I apologize for those so extremely simple questions.
-- 
Santiago Muelas
Profesor de Resistencia de Materiales y Cálculo de Estructuras
ETSI de Caminos, Canales y Puertos (U.P.M)
smuelas at mecanica.upm.es		http://w3.mecanica.upm.es/~smuelas