FC-PGAx is Celeron, PPGA is true Pentium.
Celerons are feature limited lowcost versions, a lot slower than their true counterparts.
Good enough for desktop machines in most companies or for people who just want to browse the web and maybe write some letters, but not for highperformance games or graphics (for example).
I may be wrong but all I found to be different is that the FC-PGA2 processor has an (IHS) Integrated Heatsink Spreader to assist in cooling. Oh, it's not a celeron, it's a Pentium 4 3.0GHz 512kb cache 478 pin.
Wanted to add this: FC-PGA stands for (FLIP CHIP PIN GRID ARRAY)