Will a BFG GeForce 9800 GTX+ OC / 1GB / PCI Express 2.0 x16 / Graphics Card Work With A P5N-SLI Motherboard?

That card is absolutely compatible with that board, no problem. Your 550W power supply should be enough to power it, but if you decide to go SLI later you may need to upgrade the power supply. The motherboard is already SLI ready.