Thanks for all those answers!

The point is that the m0n0 is located at a school (in Germany) which means 
that it doesn't matter how much power is comsumed. Also the heat is not a 
point for us since the m0n0 is located in a cool room in the school. Only on 
peak summer days it becomes around 30°C. Normaly the room has about 15°C. 
Guess thats cold enough to keep the hardware cool.

Since the Dell PowerEdge 2300 doesn't have a IDE Bus I would have to run m0n0 
from the SCSI Drive. CD/FD is a little to insecure, since we already had a 
m0n0 crash as a result of a defuct floppy.

But anyway, since Lee said that the CPU is used alot, although we don't use 
the traffic shaper (at least at this point) or VPN, it seems logical that the 
CPU is used while processing a lot of traffic.

Another important point was mentioned by Lee: The Server is optimized for Disk 
I/O. But that is really not the important part on a m0n0 Router.

I've also considered Daniele's and Chris' statements about the heat of the 
Computers but that wasn't a problem in the past.

Since I'm only a student at my school and do not have direct access to the 
hardware without my teacher, though I'm maintaining the m0n0 ;) I wasn't able 
to provide more information about the hardware quicky, this is what else I 
know:
P4 Chieftec Powersupply
P2 Some ECC Memory

I know this information is rare but after considering the mails you've send 
and having a lil discussion with my teacher we will go for the P4. I will 
probably get some more RAM although it wasn't needed so far. And maybe an 
extra fan ;)

Anyway thanks alot for all your responses! You helped me/us alot!

Greetings
 Alexander

PS: About the power consumption: 
We got a 
IBM x35* with 2x 600 Watt (Can't remember the last digit of the model right 
now)
Dell PowerEdge 4300 with 3x 320 Watt (IIRC)
2x P4 with each 1x 360 Watt
Powersupplies, I know I cannot simply sum those up, but still, I guess there 
will be some consumption caused ;)