Hi Jody,
Adam and Franklin have given you good advice.
It sounds as though your G5 has PCI-X slots so you should have no problems using DeckLink cards. If you want to make sure, please see the following technical note:
https://www.blackmagic-design.com/support/detail.asp?techID=76
As Adam mentioned, if you simply need to connnect to a Digibeta, then the base model $295 DeckLink card is all you need. It also has a spare SDI monitoring port which you can connect to an SDI monitor. These monitors can be expensive and many people prefer to use low cost analog video monitors. That’s why we make the $595 DeckLink Pro which is similar to the base model DeckLink card. It can connect to your Digibeta but also has analog video monitoring output. This can be switched between YUV component and composite analog video output. This card also includes genlock in case you need to lock to an external reference signal. My guess is this would be the card for you.
Your Mac is also capable of HD work. If you are going to work in HD, then the equivalent HD cards are the $595 DeckLink HD and the $1495 DeckLink HD Pro 4:2:2 respectively.
In regards to potential interference from your graphics card, this is not an issue for digital connections such as the SDI connections between the DeckLink card and your deck. It is only an issue for analog components and could potentially affect the analog video monitoring output of the DeckLink Pro card. Graphics cards from NVIDIA are shielded and do not cause interference with analog components. There is a technical note about this at https://www.blackmagic-design.com/support/detail.asp?techID=66 which should help you to avoid any such issues.
Regards,
Luke Maslen
Blackmagic Design