im not sure about requirements but i run it on a P4 3 gig with 2 gigs of ram. Including spam probably about 400 transactions a day with absolutly no issue. Not to mention its the DC DNS HTTP BES and FTP server.... with 15% sys resources used..
3Ghz single core or any of the dual cores with a couple gigs of ram will be fine for a 20 user Exchange server unless your a spammer. I'd also suggest putting it on a RAID array.