I love those books.
Fred Saberhagen actually lives here in Albuquerque, I hear.
The major problem I see with converting the swords to D&D is you'd have to keep them more powerful than the gods, or they'd lose their flavor, and work out how they were created (by Moradin? there isn't a D&D smith god). Having characters running around with a weapon that can affect/kill a god could potentially be dreadful.
Can you imagine what would happen if a PC got a hold of the Mindsword? <shudder>
And in a world with clerical magic, Woundhealer loses a bit of it's coolness. What good is a wound healing sword when any mid-level cleric can surpass it and bring back the dead?
Also, from what I remember, some of the swords are poorly explained in the books: Soulcutter and Doomgiver, for instance.
Anyway, they would all be at least +5 Keen Vorpal bastard swords. Perhaps higher.
You'd have to make up D&D conversions for all the swords and stick by the letter of the rules you make. The swords are powerful enough not to allow saving throws for any of their functions (or maybe DC500 or so?), unless you have a sword that counters that particular function.
examples:
Shieldbreaker
The wielder of Shieldbreaker must use Sunder on all weapon attacks against him as a free action, automatically destroying any such weapon, including other Swords of Power. There is a 50% chance of also destroying the armor or shield of the attacker. The wielder of Shieldbreaker also gets a free attack on any opponent attacking with a melee weapon and within reach. This free attack takes place immedietly after the opponent's weapon is Sundered and, if successful, is always a critical hit for maximum damage.
All spells, spell-like effects, and supernatural abilities that target the wielder of Shieldbreaker automatically fail, including those from other Swords of Power. Area of effect spells that include the wielder in the area also automatically fail. Any Sword of Power that would normally grant a bonus to any roll versus the wielder of Shieldbreaker does not grant its bonus.
During combat (after initiative is rolled) Shieldbreaker fuses to the hand of the wielder, and can only be dropped or thrown away if the wielder makes a strength check DC 25. The DC increases by 2 points for every continuous round of combat, and decreases by 10 points for every round of no combat (if combat resumes, the DC is always at least 25).
The wielder of Shieldbreaker take 1 point of strength damage every 5 rounds it is wielded. This strength damage is restored at a rate of 1 point per hour after combat ends. If the wielder loses all their strength points, they collapse and Shieldbreaker falls from their hand.
Shieldbreaker cannot harm an unarmed opponent, no matter what (natural weapons are not considered weapons in this interpretation), though it will still fuse to its wielder's hand if held during combat.
There is one counter to Shieldbreaker: Attack unarmed until the wielder loses all their strength, and hope they don't make the DC to drop the Sword.
Farslayer
If Farslayer is held and the death of a specific individual fervantly wished (the name of the individual is not required: "The man who killed my brother" would be close enough), the Sword will leap from the wielder's hand in a rainbow streak that can penetrate all barriors, magical and mundane, and ignores any armor or protections of the specified target. The sword implants itself firmly in the chest (or equivelent "middle") of the target and instantly slays them beyond all hope of revival. Even a True Ressurection spell or direct, divine influence will be unable to reverse such a death. Farslayer can reach any plane of existance and is capible of slaying divinity, reaches any target within five minutes of being sent.
The counters to Farslayer are 3:
If the target is wielding Shieldbreaker, Farslayer will be Sundered and destroyed forever.
If the target is impaled with Woundhealer before Farslayer strikes, he will not be harmed.
If the target is wielding Doomgiver, Farslayer will instead return and slay the person who sent it.
edit: added the str drain of Shieldbreaker.