I'm sure I already know this but the 2600 has 32 and 64-bit EFI compatibility?You can't make the GeForce 7300 GT work on the new Mac Pro "2008." It's not like you can install a driver in Mac OS X. It lacks the 64-bit EFI code in the card's firmware.
If you need an affordable second card, a much better choice would be another Radeon HD 2600 XT at $150.