well WSS and IIS6 should be on the same box... makes life easier .... WSS runs off a MSDE backend IIRC(correct me if I'm wrong) so puting all those on the Dual Opteron along with SQL2005 would be easier to maintain...(especially if the DB gets hammered a lot giving you the flexability of per CPU licensing)
Putting the mail on the Xeon would be the only thing left to do......
If it were me and I could talk the client into SBS (if they qualify) that would be a better choice(exchange, ISA, etc etc etc)... then pull the ram from the Xeon and send it back (

sorry ima AMD fanboi) ........
just my input