The Corsair has many more amps on it which is what you need to run SLI. The Goodpower will not SLI 8800GTS's (unless you are incredibly barebone and stock)
Basically the Corsair 520 has more amps for like a quad-core and gts if needed. The Goodpower has enough to run a dual core and 8800GTS overclocked
EDIT: The company you bought from probably told you to get a 750w becuase even the CRAPIEST 750w better **** sure have enough amps to run a good card. There are 550w PSU with NO AMPERAGE and horrible stabaility that you could buy...if its for SLI then most 520w PSU's cannot handle it but the Corsair is an exception as its extreme quality. Trust us over the company who knows not to much