Q: Can I add this card Apple ATI Radeon HD 5770  to my existing ATI Radeon HD 4870 card?

Yes, or you could replace your current card with it and still get twice the performance, even driving three displays.

You WILL need to deal with the Aux power cords. I believe each of the cards you mentioned requires ONE cord, and there are two outlets on the motherboard to power them.

Drivers for accelerated operation are in 10.6.5 and later.

You cited an article about adapters to NTSC composite Video (yellow RCA jack). Is that really part of what you are doing?

The 5770 will be faster even if it is driving three displays. Be sure to get the Apple-firmware version, not a version for a PC.

Your Mac will get fairly HOT driving two cards like this and the fans will come on more and be loud. There is no advantage to this.

If you are literally driving three displays with a 5770, you must take special steps to get the third one to light up.

Here are some benchmarks from bareFeats. he is not convinced the 5770 is so much better:

If you already own a Radeon HD 4870, there's very little reason to upgrade to the 5770 kit Apple is selling

http://www.barefeats.com/wst10g4.html

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Check the charts in that BareFeats article. It turns out he did exactly the tests you are looking for, featuring 4870, 5770, and 5870, and one more.

The card you cited is an Apple-firmware card, and the generally most economical source (unless you live in New York, where the total tax is very high).