What brand is your hardware.
I don't have experience w/ a softPBX, but for our 3com VOIP hardware nbx, the nbx just has to sit somewhere on the same network as the IP phones.
THink of each IP phone as a seperate node on the network, each phone has an IP address/subnet mask/default gateway but ALSO has an entry where you must assign the IP address of the PBX (or NBX). With 3com IP phones, you can actually daisy chain a PC off of an IP phone so that you don't need additional ethernet ports at everyones desk.
As long as that PBX is on the network, you're golden.
The type of hardware you going to need to run the softpbx depends on the brand of the software and the load you are going to put on the phones. Of the 50 employees, how often with they use the phones? Do you have telemarketers make calls constantly for 8 hours a day?