I would go for the Radeon 9600 XT.
It is slightly faster, and like Merlin said, you can overclock it easily with its own Overclocking-tool which is called "OVERDRIVE" and is found on the main control panel.
The FX 5700 Ultra isnt that much worse, but overclocking would be more of a challenge, and it is probably noticably slower than the 9600 XT.
Your choice though! GL on building your PC!