Not much. In his first season, Beckham assisted Ruud 3 times. In Ronaldo's first season with Ruud, he assisted him 3 times. Yet somehow Ronaldo is remembered almost as his nemesis, while Beckham was his ultimate partner. Pure myth. Giggs was a much more productive partner for Ruud, Yorke was a much more productive partner for Beckham.
In Ruud's first season we scored 87 league goals, then 74, then 64, then 58 and finally in his last season 72. That low of 58 goals in 04/05 came mostly without Ruud, but the 64 goal season in 03/04 was our worst goalscoring return in PL history at that point. In the same season we conceded just 26 goals, our (joint-)best defensive record in PL history at that point.
Our attack with van Nistelrooy was worse than the 5 years prior and the 5 years after. The way he played was clearly a part of that.