Boromir I

[Dakêsîntrah]

The wisdom of the will if far superior to skill in arms.

[Pitchwife]

I'm with Findegil and Galin here about strength of will being Boromir's forte against the Nazgûl.
Concerning his death, the draft versions of The Heirs of Elendil in HoME XII read:

A: Death hastened by wounds got in the war.
B: His life was shortened by wounds received from the poisoned weapons of Morgul.
C: His life was shortened by the poisoned wounds he received in the Morgul-war.

So I'm inclined to read the 'Morgûl wound' of the final text in Appendix A as abbreviation/condensation of the wordings in B and C above.
Besides, if he'd been wounded with a Morgûl knife like the one the WK used against Frodo on Weathertop, wouldn't he have been turned into a wraith rather than killed or letally poisoned?