DM-turned-GM seeks advice, general and specific... Options

[Pongolyn]

I'm a longtime D&D dungeon master with a group currently making the transition from d20 to Shadowrun, which I'll be game mastering for. Our group varies in experience from "never played" to "GMed a few games" (I'm in the middle, having played 2e once several years ago but fairly familiar with the genre).

I've read some of the previous posts on the differences in the game system, and they're very helpful. Right now I've got some prewritten material coming in the mail (First Run), and I'm trying to build an interesting game setting to write some original runs.

We're starting out in early 2055 in Seattle (we eventually want to run Super Tuesday so D is still alive). My normal practice when preparing for a game is to create a base of NPCs with some background and a few key locations that the players are likely to interact with, and let the players drive the plot as much as possible.

When you create non-player characters (as contacts, enemies, whatever), do you generally use the prewritten archetypes "as-is" and merely write different background stories for Joe and Jane Decker, or is it necessary to customize their stats to make them unique? Do you assign Karma points (D&D: "experience") to your NPCs when generating them, and if so, how many is a fair number?

Are there any other tricks or routines you go through in a game's beginning stages to prepare yourself? Any advice you have to offer would be appreciated.

Pongolyn