You could try to base it off of the WEG Star Wars attack/damage system -
To hit - d20 + BA + Dex Mod + magic/feats/misc.
To defend - d20 + BA + Dex Mod + magic/feats/misc.
To damage - weapon damage dice + Hit Die type (represents not only toughness to absorb damage but also to dish it out - need something to offset Hit Die resistance) + Str Mod + magic/feats/misc.
To resist damage - Hit Die type + Armor Bonus + Con Mod + Natural Armor/feats/magic/misc.
If damage roll is equal to or greater than the resist roll, consult chart -
0 - 3 = Staggered (character staggered for rest of round and next round, multiple Staggered results only increase the amount of time Staggered until the character has suffered a number of Staggers greater than his Con Mod + 1, at which time he is unconscious for 3d10 minutes - character must rest for a turn to return the number of Staggers he has suffered to 0)
4 - 9 = Fatigued (second time results in Exhausted)
10 - 12 (or third 4 - 9 result) = Disabled
13 - 15 (or second 10 - 12 result) = Unconscious
16 + (or second 13 - 15 result) = Dead
Each of these conditions is covered in the DMG (pgs. 83 - 85).
The ranges can be tailored and such. Healing rates will have to be decided (rest so many days, make Con check, if in this range (11+) improve by one step, if in this range (5 - 10) stay the same, if less than this (< 4) get worse by one step).
Anyway, most of my players were not amiable to a more realistic game and so I did not pursue this too much (I was working on a classless/levelless/hit pointless system that still would be compatable with the D&D books I have - feats, weapon skill (BA), skills, armor prof., saves, attributes, etc. purchaseable with skill points, which are rewarded instead of experience).
I just thought that I would throw that out in case anyone would like to run with it.
Later