If you can beat it non-lethal then you certainly don't need a walkthrough for lethal. It's
much easier. I preferred to stay at range and cap 'em from a distance.
Do you want to be lethal/stealth or lethal/"balls-out"? Do you want to stay at range or get up close and personal-like? These answers will govern what skills you take.
For a general "good time" though, SELL OFF your starting pistol skill and dump as much as you can into rifles. Can't go wrong with the game-pwning rifle skillz
And I am curious: How is it that you are playing this game? Has it be re-released somewhere?
You can buy it digital distribution like Steam and such. I still have my original disc. Such an amazing game.