300 Mhz proc
256MB of RAM
cd rom and all that good stuff of course
if possible, a single HDD for system/exchange program files, a RAID1 array for each set of tranaction logs, and a RAID 5 array for each database
the disk setup would be for ideal situations of course....but you should defiantely at least haev the tranaction logs and database on a seperate disk.
not sure if its in your budget but....
I found that a good 1.4 intel server with 750+MB RAM and the above disk config makes a damn good setup