To migrate from SQLExpress to SQL 2005 is fairly painless though it does take some MS SQL knowledge.

There are a handful of ways to do actually accomplish it.

The point that matters for you right now is that you can take the entire database from SQLExpress, and make an exact copy on MS SQL 2005. There will be no lost data, and you will not be prohibited from growing or eveloping in SQLExpress in the meantime.

When you do migrate, you'll have to also tend to the issues of user accounts and connection strings etc.

Essentially, all in all, an easy task that an experienced MS SQL dba or informed user could accomplish in a matter of minutes. You'll just need some guidance or assistance when the time comes.

Note that I'm talking about an actual install of SQLExpress and not the file that accompanies DNN which I never use.

I prefere SQL Express for development machines, and SQL Server 2005 for production machine.  If money is an issue then there is really no reason to not use SQL Express to start, since you can migrate to SQL Sever 2005 when the traffic to the site justify it.  More hosters support SQL Server instead of SQL Express, so if you are going to be hosting the production site your hoster might only offer SQL 2005.