which FreeBSD?

[David Newman]

Greetings. For a small office (~10 users), I am planning to build a mail and
DNS server using FreeBSD-SMP; details below. My requirements are
availability and performance, in that order.

Which FreeBSD is better suited for this -- 4.8 or 5.1?

Hardware:
Compaq Proliant 1850R 6/550 (2 x 550-MHz PIII), 1 Gbyte RAM, 2 x 18-Gbyte
SCSI drives

Software:
postfix, courier-imap, bind

Thanks for any guidelines on this.

Regards,
David Newman