What server for what role?

I currently have 1 Dual G5 2.0 GHz xServe running 10.4.11 for 37 managed macs, 4 windows boxes, and 57 email accounts. All mac users have network homes. I am buying a new 2.8 quad core xServe w/ 10.5 on it. I was wondering what you all thought should go on where? I mainly bought it to seperate mail from network homes. Do you recommend mail on the intel xServe, or putting authentication/dns/dhcp/afp on it? I could go with any combo there as well. The network homes and mail are located on an xserve raid via fiber. If needed I can also put the G5 xServe on 10.5 as well. Thanks.