The technical article on migrating Windows NT to SBS can be found here:
http://www.microsoft.com/windowsserver2003/sbs/techinfo/productdoc/default.mspx
Note that one of the gotcha's here is that you will need NEW servers to run the SBS.
Another gotcha in your plan is that both of the SBS servers are going to want to host the Exchange organization, root domain of the AD forest, DNS Start of Authority, etc. You should note that you can have only ONE Excahnge organization per AD forest with Exchange 2003.
In addition, you will not be able to use the Windows 2003 Domain Rename feature with SBS. Once it is set, it's locked in.
Another gotcha (potentially) is your Exchange server most likely has a SMTP domain which is accepting mail off a MX record for yourcompany.com, unless you are part of a larger organization and they will be holding it.
My advice is if you want to move to Windows 2003 and Exchange 2003, but cost is a major concern, do the following:
Install a Windows 2003 server that will replace the NT 4 PDC. Leave the Forest and Domain in Windows .NET Interim Functional Level. This will allow for only Windows 2003 and NT 4 domain controllers, but that should not be an issue in your enviornment.
In the second office, install a Windows 2003 domain controller. This DC will be in the forest as a peer DC to the root at the other office.
This will allow you to maintain your NT member servers until such time as you can upgrade them.
If you choose to move to Exchange 2003, install 2 servers, one in one office and one in the other. This would allow you to easily migrate the Exchange 5.5 accounts to the Exchange 2003 organization.
