first of all - i dont really think that most people are going for d700s, at least not the ones that dont work in the corresponding industries
if you are looking for double precision floating point performance and work with software that supports this stuff then the d500s will work great,
in any other case they will be equally or < than the d300s which is not very remarkable i think - if you want to go safe then take the d700s which beat both of the lower end cards
for consideration:
D500 -
http://www.3dmark.com/fs/1593405 - Scores with 7300
D300 -
http://www.3dmark.com/fs/1710373 - Scores with 7900
D700 -
http://www.3dmark.com/fs/1779308 - Scores with 10700
in the end its all up to how you intend to use those cards
hope i could be of some help