The FX5700 range goes head to head with the ATI Radeon 9600 range.
The 2 premier cards in that range are the FX5700 Ultra and the ATI 9600XT.
Both card offer best in class performance, see here for an indepth head to head review:
I personally would do my upmost to spend the extra £40 and get either a ATI 9800Pro or FX5900 as they offer you double the performance of the above two and will allow much better performance with newer games.
However if it was between FX5700 ULTRA or ATI 9600XT, i would buy a 9600XT for its better DX 9 support as I like playing newer games.