Hi Dustoff,
I use FSX (boxed version) and occasionally fly MP in several flight communities with some using P3D. If you fly FSX and want to do MP cross-platform with P3D, I think you might need FSInn/FSCopilot combo and Estonia Migration tool (for P3D user). If you want to do MP with only boxed FSX users, you can opt either with FSOpen (new substitute for Gamespy) or FSInn/FSCopilot. I've also heard some using the FSCloud project (still under development) which can do MP across all platforms (FSX boxed, FSX Steam, X-plane and P3D) and requires a client download but have not used it personally. Some also report that FSX Steam version can do MP with FSX boxed with FSInn/FSCopilot as well as other FSX Steam users.
There are lots of options. Whatever route you want to take, I suggest you research on the requirements and how to set any of these up. Goodluck!
Trex3D