I tried both Red Hat 9 and Mandrake 9.1. Both within the last week. I had no probably with DHCP it was automatically detected during both installations. Are you using a built in nic card? If so I've read on other forums it tends to be a problem.
Both to me are decent distro's although mandrake seamed to have a quicker performance. Having a sluggish time in RH9 and so have other people. Even limiting processes that startup on boot down to the bare minimun hans't really helped.