Both those cards will look no different from each other, but the 9600 will be faster by a margin. Seeing as N64 emulation is almost graphics card independant (as long as it's GeForce 3 or above, your fine whichever card it is) you'll have good accuracy with both I should imagine.
The 9600 is the better choice for a video card since it is only a tad more expensive and is much faster in PC games.
As already said, a processor upgrade will boost the speed up, which is great for games that have higher requirements.