not much in the way of integration is possible with professional edition. You'll need Enterprise edition at the minimum.
For instantaneous transfer, you'll burn through a lot of api calls, so look at performance edition.
I think you'll probably need to settle on a scheduled set of csv exports every 15 minutes or so.
Actually, I just did a bit of digging and discovered that if your data on the AS/400 machine is a db2 database ... your world just got a lot easier. Tools like Jitterbit, or Informatica can link directly to Salesforce AND ODBC / JDBC databases. You'd have to contact the Jitterbit/informatica guys for details but I think buying one of those tools (which will cost you more than the GDP of Zimbabwe, beware!) will allow you to directly connect them and schedule periodic syncs of data.