First off, for a mail environment that small, I'd suggest that you probably don't want to do a co-existence migration, but more likely a cutover. It'll avoid a lot of PITA and extra work to just get the new environment stood up, transfer your data and cutover your MX records, DNS, gateway settings, etc.
Secondly, a smarthost is used in mail migrations to make sure mail arriving at one system for a user on the other system actually gets to the user instead of being bounced back. So, if this were a larger, typical migration, you wouldn't be able to get it all done at once, so you'd move your users over to Exchange 2010 in batches. Once they're moved over, you need to send/receive connectors in place (smarthosts), so that when mail for the migrated users arrives at the 2003 Exchange server, it checks the 2010 Exchange Server to see if the recipient exists there before bouncing the message as undeliverable.
Finally, I'd recommend you do some more extensive planning and testing before actually embarking on this project. It's not something you just want to "try" and hope you get right - not that it's all that hard, but you need to get a lot of little details right.
Microsoft (Technet) offers some good guides and general scenarios on Technet for mail migrations, and the MS Exchange Server forums have a section devoted just to this topic. I'd check there (and in fact did, during my last mail migration project). Of course, like I said, for only 50 mailboxes, I'd recommend doing a cutover one weekend.