I'm currently brewing my own Fallout system and it'll most likely end up as a hybrid GURPS/D20 system.
For a D20 system try Glutton Creeper Game's Exodus which used to be the Fallout d20 before Bethesda got out the Lawsuit stick it has useful stats on weapons and the flavour is pretty much identical with slight name changes.

GURPS impresses me with it's flexibility and simplicity here but I have yet to find any actual pre-made post-nuclear apocalypse flavour anywhere. The tech levels in GURPS also vary wildly from that of the Fallout universe so tweaking there would be necessary.

For d20, the SPECIAL system already exists.
Strength = Strength
Perception = Wisdom
Endurance = Constution
Charisma = Charisma
Intelligence =Intelligence
Agility = Dexterity

Oh yes, Luck is missing. The mechanic I came up with was that players roll their roll luck each morning (either 3d6 or 1d20) and could take perks to flat boost it up or allow them to reroll the result. Players could then either choose to add their luck to a roll or test their luck against a skill challenge/check of some kind in place of the usual roll. I'll bump up my thread once I get some of them more hard and fast rules set up.