Don't know that it makes much difference which you upgrade first the clients or the server, but:
Double check your server's hardware stats against the system requirements for FMP 10 Server to make sure you won't also have to upgrade your hardware.
Check out KB 7152 FileMaker Server 10 and FileMaker Server 10 Advanced System Requirements for the official word.
I had already assumed I would be using new hardware, or at least taking the opportunity to upgrade the OS.
Thank for the tips.