First, I was curious why you were upgrading to Exchange 2k3? Is Exchange 2000 not working well for you? I personally wouldn't upgrade unless you were also upgrading your AD to 2k3. Also you WILL want 2gigs of ram. More than that won't make much of a difference (in Exchange 2k it can even cause problems).
What kind of mailbox limits do you currently have? I wouldn't be too worried about performance if you only have 75 users unless they treat their mailbox as a storage medium. My users are limited to 20megs except in special cases, and we have a 6 month retention policy. Like others have said, I would try and "downgrade" the machine a bit unless you really are getting stuff for free. I really wouldn't want to have MS SQL server running on the same box as my exchange so look into getting another cheap box in the future.
I would also look into Dell's Gold support with 4 hour response time. It may cost a little more, but there's nothing like having a raid controller go out and then getting a replacement in less than 90 minutes. I even had a poweredge 1650 burn up next to me while I was doing some updates and I had a tech out looking at it within 3 hours. (They wanted to see what happened, we got a 1750 replacemnet, and Dell ended up recalling the mobo on the 1650.)
I almost forgot... I would highly recommend getting the Exchange connector for Veritas (unless you are using a really old version then I don't know what's available). Technically you could use Windows backup to backup your Exchange server, but as someone mentioned earlier, if one person needs a single message or even an entire mailbox restored you will need to restore the entire information store. This will not only take a long time, it will also replace ALL email with the last backup. The exchange piece for Veritas lets you restore pieces of your information store let it be mailboxes,messages, contacts, whatever.
