It seems to me that The Dude is only using one core on my computer, any fix - workaround known?
What is your operating system? The system that you are operating. 
How does that matter?
Anyway, I am using Windows XP pro on AMD 939 dual core 2ghz
Different OS’s handle multiple cpus differently.
The OS is using both.
Does the Dude use dual core processors?
Probably the Dude is a single threaded program, so it will not use more than one (logical) CPU.
But that not a problem whatsoever. It doesn’t need much CPU power, so it would be not beneficial at all to put in all the trouble to make it a multi-threaded program. It wouldn’t gain anything by it.
But I intend to try to SNMP monitor over 400 devices.
Can I estimate CPU, memory and network usage before I try that?
That computer will be more than adequate to serve as a Dude server.
I have approximately 475 devices on my Dude server presently and it is a 2.8GHz P4 with Hyperthreading (Intel fake dual core). Typical CPU usage is around 4-6%. This is with 6 active clients also accessing the server.
Memory usage is around 75M for two Dude services and admin client running on server. Have 1G of RAM in server.
Network usage averages around 500kbits/s. Network usage can be adjusted by adjusting the polling interval.