Hopefully this will answer your questions...
1) Best way to ID the chipset is by ID'ing the motherboard then visiting the mobo makers website. Most mobo's have the model name/number printed right on the PCB.
2) nForce refers to nVidia's motherboard chipsets whereas gForce refers to their line of graphics board chips.
3) Depends on the specific chip, but the MCP in MCP55P means Media Communications Processor and the P after the 55 I believe refers to the revision and/or features of that partcular chipset.