Welcome to VGChartz by the way! The PS3 is a very powerful console as I'm betting that the console probably won't be used to full capacity in about 3 years from now! As for MGS4, I would say that would use about 40-60% of the PS3's capacity.
Generation 8 Predictions so far.....(as of 9/2013)
Console that will sell most: Nintendo Wii U
Who will sell more consoles between Microsoft/SONY: SONY