Exchange standard has a limitation of 1 single storage group, 1 mailbox
store, maximum size 16GB.
I think that will be just fine for 200 users.
The best advise I can give you has to do with file location. If possible
install Exchange Logs and DBs on separate disks:
Logs: RAID 1
DB: ideally RAID 0+1, if not possible, RAID 5

At the end of your installation you can run Exchange Best Practices Analyzer
Tool (www.exbpa.com).
There's lots of good documentation at the Exchange Server 2003 Technical
Documentation Library
(http://www.microsoft.com/technet/prodtechnol/exchange/2003/library/default.mspx)