d20 Modern.
It was a blast, if you can capture the 'neogrit.' If you can afford to import a few soundtracks, that does wonders for the feel, too...try playing a really slow track during an epic battle, and see where it takes you.
For the cowboys:
* Spike: Fast Hero/Martial Artist
* Faye: Charismatic Hero/Personality
* Edward: Smart Hero/Techie
* Ein: Smart Hero
* Jet: Tough Hero/Soldier/Investigator
-------
Setting-specific Advanced Classes:
* Soldier [d20 Modern]: For the wars that have ended
* Martial Artist [d20 Modern]: For the fancy fisticuffs
* Gunslinger [d20 Modern]: For the fancy gunfights
* Infiltrator [d20 Modern]: For syndicate spies or special agents
* Daredevil [d20 Modern]: For your average bounty hunter
* Bodyguard [d20 Modern]: For your noble thugs
* Field Scientist [d20 Modern]: For techies; ship-makers, gate constructors
* Techie [d20 Modern]: Hackers and inventors
* Investigator [d20 Modern]: For your typical detectives
(Field Medic: N/A, though it wouldn't hurt anything, I suppose)
* Personality [d20 Modern]: For those who can get anywhere (they don't have to be celebrities)
* Negotiatior [d20 Modern]: For smoooooth talking.
--------
A few other ideas:
* A "Desperate" feat: +4 bonus on a [wealth] check, but -4 on the one after that
* A "Syndicate Member" Advanced Class full of organized crime goodies
Other than that, d20 modern is the *perfect* system for a Cowboy Bebop flavored adventure.
