been running the same system (mac os x 1,.6.8) with a phoenix PHX-D20HDSDI, just use only 1 input for full HD and with the latest drivers, I think you can use 2x 720p aswell, but never tested it on that system, there's some minimal delay which you can see in close-ups but it all depends on what you're trying to do, buy from simon or nev, a faster server will give you more margin though, try testing it with your setup

buying the 2 inputs card will be more economical if you would need those in the future

have no experience with other cards except LFG's, works aswell