You can of course use the ATI5770/5870 just on it’s own as well. You don’t need a GT120.
Please do note the limitations of performance, and you are limited to HD resolution files and timeline.
It might be better to get the 5770 instead so that you can upgrade to a Q4000/Mac later when you feel you need more performance, or need to process higher resolution images. The 5870 takes up both the power connectors, and there’s none left for the Q4000/Mac.