Hi there,
There are a few guides out there for BizTalk high availability and load balancing. The BizTalk Operations Guide has a great section on high availability which at the same time gives you load balancing:
http://msdn.microsoft.com/en-us/library/cc296682.aspx
In a simple explanation, BizTalk load balanding is provided by having its server components installed on more than one server that are part of the same BizTalk group – the first BizTalk installation you create the group and the BizTalk databases, and on the other server(s) you just join that group. BizTalk then has a central SQL Server database (the message box database) that all the BizTalk servers in the group look at, and then any of the servers can pick up pick up work to do from the message box database. You don’t typically need to cluster BizTalk servers for it to be load balanced. You do cluster the SQL Server hosting the BizTalk server databases though for high availability. MIrroring isn’t supported for BizTalk database backups because BizTalk applies distributed transactions that span across multiple databases. For messages being received via HTTP you need to load balace IIS on the BizTalk servers involved as well, but it sounds like you already have the switch for it.
What servers to use would depend a lot on the existing load of your servers – you could consider installing BizTalk on your 2 application servers, but only if they are very under utilized.
I’d recommend contacting Microsoft or a Microsoft integrations partner for some input on your specific situation.