i use several of the apple active adaptors with the long names that always confuse me, and ive not ever had a problem with one ever. i use two 5770's with 4 adaptors to run 6 30" acd. i also have a mbAir with one of the adaptors to run an external 30" acd, no problems there either. i recall seeing the adaptors on ebay for like half price, 45 usd or so.
regarding your question, a second graphics card (5770) would still require one adaptor for the third monitor, as only the first monitor on each card does not require an adaptor.
i would recommend just the one graphics card and the two adaptors, due only to my experience that when i move video (ie:an itunes movie) from one card to the next it gets jerky. not sure why it does it, but i have enough displays so that i can move video around as i need to and still stay on the same card. if you only had 3 displays on 2 cards you limit yourself abit, assuming that your setup would also see jerky video when moved from card to card. i'm not familiar with your setup, and i'm no pro, but this is just my experience, with apple displays and 5770's.
there may be advantages to multiple cards, and jerky video may not matter to you. i just restart the video on the other card and it's fine.