I used the AD725 for my RGB circuit, works great. The main difference w/ the 725 over the 724 is that the 725 uses an oscillator to get it's signal where the 724 makes you basically build an oscillator circuit from a crystal. It's not that bad, but I liked the 725.
Also, build in the Y-Trap function of the circuit, it's a filter that REALLY improves the Composite video quality. S-Video isn't affected by it, but that is already good as hell.
Good luck with the circuit, although I reccomend you use a WIDE Surface Mount adapter to get the thing to integrate into a circuit easily.
You can see my final circuit (minus the Y-Trap filter, that's on another small board) here:
<img src="http://web.njit.edu/~jal0737/GFX/Neo-Geo/RGB%20converter.jpg" alt=" - " />