I haven't played D3 on my 9600 yet, but from what I've heard a 9600 can be sufficient (and a 9700 is just an OCed 9600). However, there are a couple of features that determine how smooth it will play:
Video Ram: 64MB isn't very sufficient, but will get the job done. 128MB is better
RAM: Whether you have 64MB or 128MB VRAM, I've heard that having 1GB system RAM really reduces stuttering.
HDD Speed: I would imagine having a 7200RPM drive would also ensure responsiveness in the game, as some data will probably have to be cached to the HDD.
If you want to be serious about D3, spring the extra monay & get 1GB. Otherwise, you're good to go! Enjoy!