Of the two, the FX1700 would be better.
IMHO, they are both a waste of $.
The FX1500 is similar to, but slower than, an old 7900GS and the FX1700 is similar to, but slower than, an 8600GS.
The only advantage they hold over the standard versions would be the driver optimisations which can be made to run on most any consumer card.
You can locate reviews containing both cards Here and Here.
If it were my cash, I would skip on the 'pro' GPU.
With an i7 920 and a 4770/4870/GTX 260 you will get much better performance than a slower CPU and either of your suggested cards.
Once you soft mod your cards to run the FireGL/Quadro drivers, you will see another performance increase.