games that are ported to the PS3 will naturally look the same on the 360 because the it's a port. Like you said, first party games look better, yes, of course, but the only reason shared games look the same is because most are ports to the PS3 instead of ports to the 360.
1 is deactivated automatically. It has 1 ppe (power processing element) and 6 SPE (Synergistic Processing Elements). So yes, it uses every core available to it when the game programmers decide to make it so (Uncharted 2 for example)