Don't use SBS but....
I have a couple of clients on Windows 2000 AD FSMO servers running at 2.8 ghz (Supermicro board), Lsilogic u320-2 raid5- 200gig arrays, 2 gig ram ,with Great plains Dynamics/ SQL 2000, the servers are general use file servers.
Great performances, DNS and DHCP respond quickly, will run for months before I shut down for MS patches. Servers are also running Symantec CE 8.0, Executive Software Undelete and Diskkeper, Veritas 8.6, APC battery backup. SQL is set to use 1.1 gig of memory
Having a U320 interface is important, disks are Seagate 15k drives, 10k drives are acceptable.
I consider this the maximum load I want on the servers. Placing Exchange on such a server would be a heavy load, no less a mail server should not be on a general use server due to security and virus threats. Personally I would not even consider Exchange on a SQL 2000 equipped server.
Aside from Exchange being a resource hog, just how much do you think one server can handle. On the servers I manage, the network card is basically the only bottle neck , not that it's performance is poor by any means; the fact SQL basically does all the work internal to the server makes up for the network card bandwidth. If you install Exchange on it, the added network activity/resource use will slow down every thing.
Personally with any mail server, I require anti spamming software, and more than one AV scan engine, which is quit a load.`
"As I understand your responses, in theory we can probably run SQL and Exchange on the same server if we have enough memory." Theory...Microsoft is full of theories ( bullsh*t). Just like they have hyped raid 0 ( beyond dangerous), or and as in previous times they touted how you could have a disk array of 32 drives, of mixed interfaces, mfm, scsi,ide, and have it remain intact for more than 5 minutes ( A Ms wetdream).
Your going to like Dynamics on SQL, my client's systems were ported over from Dynamics/ Pervasive, which was an absolute nightmare to maintain.