Clusters are only really good where the Exchange Servers are at the same location (so they can access a SAN for shared storage). While you CAN do clusters over a VPN link, it's the last thing I would want to try, when there is an easier option out there. Plus, you can't cluster with your current equipment - all nodes in a cluster must run Windows Server Enterprise Edition, and when it comes to Exchange, only the Enterprise Edition supports Clustering. This advanced feature support in each version is quite obviously reflected in the price of those products.
Perhaps a much easier option would be a product like DoubleTake. It is one of the leading third-party products in the industry for managing replication of Exchange Servers, as well as SQL Servers, File Servers and many other things. One of the main advantages is you configure the replication with a few simple clicks (nothing as complicated as a cluster), and the DoubleTake software handles failover and everything else automatically. It is, of course, not a Microsoft standard software, but for the price when compared with the thousands of dollars a cluster will set you back (both in hardware and software), the product is unbeatable. See
http://www.doubletake.com/english/products/double-take-windows/Pages/default.aspx.
-tigermatt
