The limitation for SQL Express comes from the Microsoft side, not from Altiris. With SQL Express you'll be limited to
1CPU
1GB RAM
4GB of database size (with SQL 2008 Express and previous versions), or 10 GB (with SQL 2008 R2 Express) as you can see in the following link
http://blogs.msdn.com/b/sqlexpress/archive/2010/04/21/database-size-limit-increased-to-10gb-in-sql-server-2008-r2-express.aspx
As for if this is enough, this is something difficult to evaluate without more data from your side. It will depend on what components of the CMS you'll be using.
As a first approach, you can try to run your environment with the Express edition, and, if you feel you need "more power on your DB", you can install a "full" edition of SQL, and migrate the DB from the Express edition.
If you use the SQL Express edition, you'll have to adjust your environtment to try to leave the heaviest DB tasks for non working periods: for example, you can configure the agents to ask for pending policies/tasks/jobs just once a day instead of asking hourly; you can configure the refreshing of the membership to filters to run at night; you can run the AD import rules (if you have any) at night, or at weekends...
Good luck (and let us know what decission you take, and if it worked ok for you) ;)
Falquian