There is a difference between Mac cards and PC cards. Cards for Mac have an extra stuff added to BIOS (the EFI part) so OS X could recognise the card from the very beginning of boot process. The card you've bought is from/for PC, hence does not have the EFI pat added. Therefore OS X can not recognize and use it.
However there are few options to make it work:
Mountain Lion with official Nvidia drivers may give you full support, but only then system done booting (read no boot screen with Apple logo)
ATYinit kext (check netkas site) this kext is used to enable unsupported PC cards in Mac Pros, but again only then system done booting (read no boot screen with Apple logo)