I have an XFX 5900XT. It's memory is faster than most 5900xx cards. I believe it's 2.2ns. I can run it a 975mhz issue free.
IMO it's better to have less of the fast stuff than more of the slow stuff.
It depends on the manufacturer. It's impossible to say that a 5900nu is better than a 5900XT.
Clearly some of the 5900XT's possess higher memory configurations than other 5900xx cards. Couple this with the fact all 5900xx cards are obviously NV35 GPU, you can get pretty fast 5900XT's for a really low price.