"Tracy Austin is by and large a good benchmark for bare minimum in singles: 2 Slams and a brief period at No. 1, (and American). More slams, no need to be No. 1, like Hana Mandlikova. A single slam might be enough with a very, very good doubles career. But for a singles-only player, Tracy has to be considered about what it takes. Novotna will almost certainly get in eventually, but less than that, probably not."
In all fairness, though, Austins' situation is pretty unique. She came in and took 2 U.S. Opens and the #1 ranking when 2 of the best ever were in their primes and then had her career cut short by injury. If, say Iva Majoli won the 1997 U.S. Open in addition to her French title and got the #1 ranking away from Hingis for a little while, I think it would be a different story.