the 5700Ultra was the last card in the FX series, its not meant to be the best in the series, it was made after all the other ones, so they fixed the problems that the earlier FX cards had, the 5700Ultra has 256Mb DDR2 and its a very good card, ive never owned a 9600XT but that is a very good card too, but i would still go with the 5700Ultra because of better driver support, and it is a slightly better card in my opinion.
one of my friends has one:
Asus A7N8X E-Deluxe
AMD Athlon XP3200+
2x512Mb DDR400, Kingston value ram
he can run farcry on the highest detail @ 1280x1024 and he never gets lower than 40 FPS.