They actually played twice, once in 2005, and again in 2006 and Nadal won them both. But, Andre was already 34 and 35 years old. Hardly a comparison to 99 when he was still in his 20's, but barely.
Also, once was on grass and once on HCs, but not sure which year was which surface.
I was replying to your comment that their match up on grass was a totally hypothetical one, and I pointed out that we do have SOME basis for a guess, they met once on THIS surface... grass (3rd round of Wimbledon 06), where Nadal won easily. Of course Agassi wasn't at his peak, but he was still playing well at that age, and of course we need to consider the fact that Nadal is a lot better on grass now than he was in 06. Their other meeting was in the finals of Montreal 05 where they had a tough match won by Nadal. Agassi was in good form that season so it's reason to believe that Nadal would be able to win many of their encounters on slower hard courts, but I would still favour Agassi on the faster ones.