to me, "now that" is closer in meaning to "nun da": Now that he has a dog, he exercises more frequently - Nun, da er einen Hund hat, ...
"wenn nun" has a different meaning, IMO, and that meaning can depend on the context. According to
http://www.dict.cc/german-english/Und+wenn+nu... it means "so what if?" Obviously that doesn't fit here. "...wenn nun die ... Titel ... kostenlos zur Verfügung stehen" would mean to me something like "... once the titles ... will be available ..."
I suppose what confused me with the OP is the "Ansporn" because there is no connection to anything in the sentence that "Ansporn" could refer to. An "Ansporn/incentive" for what? Maybe the preceding sentence is needed here for clarification. At any rate, I don't think that Strawberry's version is "spot on."
Edit: where does it say "nun, dass" in the OP, Werner?