We too have a very mature Fox app in a very niche industry ( we are the only supplier of our product other than in-house solutions ) that we are moving to another platform. One word of advise, no matter what anyone says, do not get caught up in fantasies with obscure platform technologies. MS has bet the farm on .Net technology and it is here to stay so it is wise to learn it. Sql Server is what it is the defacto standard database server other than Oracle ( which is pricey ). T-Sql is almost exactly like ANSI-Sql so that transition is easy, but learning to program for the server takes practice ( so do SPROCS ). Yeah you can learn Linux based stuff or MAC/OS but for right now and the immediate future ( next 20 or years ) MS holds the market.

We liked Fox but it had run its course both with MS and with us. We looked at several options and decided on .Net and Sql simply because we make a desktop application and .Net is a big part of that future and 100% of our clients run Windows.. We chose C# as our language. So far we are happy with our decision. The learning curve is not that hard once ( I was a C++ programmer so not that hard for me as others on my team ) you establish time to actually write code. It is not the language as much as learning where everything is as far as base classes and methods of implementation. I like to remind folks good software people are problem solvers, so language doesn't make a difference as long as it can provide the technology needed to solve the problem.

Main thing is do your homework but do get side-tracked on pie in the sky solutions that you will regret later. It took us a year to decide and we have a 5 year plan. Stage 1 database to Sql with Fox UI and middle tier ( some small chunks will be in C# ), stage 2 convert the rest over to C#.