well there are a lot of solutions to the server problem for example install the server on an other machine instead of the existing tight server eg your computer, as long as the computer is on the same lan as the clients then you are set.
Ofcourse if the current implementation with the DBase components that come with FPC covers all your requirements (which I doubt) then I do not see any reason for you to upgrade to an other database. on the other hand b-tree filer has data security and far better search capabilities build in, aka it does not require you to load everything in memory and filters data as you read them from the disk.
Now please keep in mind that I haven't touched that technology for a long time and memory can play tricks especial on events that long in the past, you need to evaluate the library your self and see if it gives you any benefit converting or not, you might even learn something new at the same time.