building video editor from older model mac

I'd like to build a decent video editor with USB 3 connectivity without paying retail. I'm told I need to be careful that the USB 3 card is compatible with the operating system's driver (hope I got that right). Currently editing on 2.66 GHz Intel Core 2 Duo MacBook Pro with 8GB of 1067 MHz DDR3 ram; want to create something faster in a tower (probably '09 and later), and add USB 3 connectivity without paying retail (oh, that apple name). On a related issue, presently am clinging desperately to 10.6.8 Snow Leopard because it's more stable and if I upgrade i have to bid farewell to my Final Cut 6/Studio 2, because I'm told it will not run on Lion or Mountain Lion. Sigh. Knowing which video card is best would help as well, though I may settle for whatever it comes with if I can get the rest right. Your thoughts, those of you with greater tech wisdom than myself (which might be just about anybody on this forum), greatly appreciated.

So are you asking if you should buy a '09 or newer Mac Pro and add a USB 3.0 card into it? I'm not sure I'm following..

Right. After a certain vintage the Mac Pro will not accept the USB3 card. Also there is a question if I want to stick with OS 10.6.8, which USB3 card will have a compatible driver.

We've had several folks with later model Mac Pros who have tried to add a USB 3 card without success. It's not only a matter of finding the drivers but also a card that is Mac compatible with your Mac Pro.