Quadro FX 1500 vs 3500 ? Which one ?

Get the FX 1700.
With Quadro FX cards, the generation of the card is more important than the model.

1500, not SLI and uses ddr2 memory 3500, SLI and uses ddr3 memory, this is why some of the numbers are faster.
SLI is not supported in SWx 2007, have not checked 2008 but i would expect that the answer is no.
if price is an issue go for the 1500 or 1700. iQ

I have long been of the opinion that the difference between a "good enough" video card and an amazingly expensive video card was very little for most ME work. When I got my new computer, I bought the cheapest card that I could get that was certified to work with SW08, the Quadro FX 550. The last time one of my colleagues got a new computer, he bought the FX 3500. We haven't done any formal benchmarking, but so far we haven't noticed any substantial difference in performance. When he spins a relatively complex model, the motion is very smooth. Mine might be just the tiniest bit more choppy. His system will spin the model as fast as he can hit the arrow keys. Mine lags behind very slightly, so that after 24 hits, I'm at about 270 degrees of rotation instead of 360. I don't believe that this slight difference makes any difference in how fast I get my work done.
You'll probably hear from other people who are very happy that they got the very fastest card. Maybe it really makes a difference for them. Maybe it will make a difference for you. I'm happy with my cheap card.
Jerry Steiger