Not to be a wet blanket, and I apologize if you see this as a thread-jack, but aren't d20 Modern rules gritty on their own? Or are you talking about location damage as opposed to "Fail this Fort save and you're toast"?
There've been numerous house rules designed to make injuries more interesting. Most of them simply add a "What did you break?" table on any Massive Damage Failure, or any time that (Double Con Score) is exceeded, regardless of whether or not you make the save.
There are also House Rules about making the Massive Damage Saves more difficult -- making the save DC equal to 10 + (amount by which the damage exceeded your threshold). This means that when you get hit by a fireball for 37 points damage, it's gonna be a tough save, and you're likely to be out of the fight.
What did you have in mind?
-Tacky