That looks like it should be a workable solution. These two possibilities may be variations of what you alrady had in mind since it looks like you’re putting up another server:

www.isaserver.org/.../...To_Publish_Your_Site.html

www.tacteam.net/.../2bettertogether.htm

Hope if nothing else it gives you some ideas or confirmation.

Setting up SharePoint for extranet isn't that hard at all. It is a
nice idea to have a seperate server  in between the firewalls except
they will still ned access  to the SQL Box on the LAN anyway. It may
just be simpler to create new websites on the Internal boxes, enable
SSL on them and publish the internal ones directly via ISA 2004 to
the internet. This a proven way of doing it and we use ourselves. As
long as you use host headers correctly and ensure the use of SSL and
lock the firewall rules down it will work fine. Also it saves you
having to create another server, Hope this helps.

The first question is:
Do you need external  access to the complete portal for your people or just
access to some information for partners?
This impacts your design.