I have a Dell rack server with a single dual-core 3.0 Ghz processor and 4Gb
of ram. Do you think that will be enough to run a VMware ESXi server with a
SUSE virtual machine running a GW mta/poa for 175 users? Would you give the
virtual machine 1 processor and 3Gb of ram?

Not (currently) planning on running any other VM's on this physical box.