>We have developped an app that runs 24/7. It is being use remotly from 3 different sites and also from HQ (although HQ uses it only for managerial purposes and is 9 to 5). The app is VB6/SQL Server.
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>Everything (front, middle and data tier) is on the network. Lately, we were told that the app is now tagged critical and that we should find a way that operations of the 3 remotes sites run at all times without any down times (except those planned for maintenance, if necessary).
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>We have looked into data replication and other solutions. But I'm not at all familiar with this kind of setup.

It's good that you're using SQL Server- depending on the version, the replication capabilities make this sort of app much easier.

Depending on the nature and architecture of the application (is this is a web app?) you may or may not have a bunch of software work to do, but you definitely have some hardware work to do.

There are several options for failover, most of them involve intelligent clustering- this can be done on by hardware (F5 BigIPs, etc) or software (Win2K's NLB, etc), but either way the software has to take into consideration the fact that there are several different physical machines that are qualified to serve a request, and so things like statelessness (or session affinity) and data replication have to be provided for.

This is really a much bigger topic than you will be able to get a complete answer for in a discussion forum. I suggest that you start to get educated on the relevant technologies by reading on the topic (the long route), or consult with someone who already is.

Good luck, and have fun.