Directed towards your second question, assuming you aren't running a real world campaign, make sure you clearly detail not only the differences between your world cultures, but perhaps the background / history of the differences.
Create cheat-sheets for players playing characters from those cultures, which you (and maybe with the help of the player) update and add to as the campaign continues - perhaps even toss in some interesting wise quotes that exemplify some aspect of a culture. For example, a saying from Japan such as "The nail that sticks up gets knocked down" as contrasted with "The squeaky wheel gets the grease" tells a lot about the importance of conformity. What about differences in courtship, marriage and achieving "honor" within the culture? Are the sexes segregated? What's up with relations with elders?
If you have non-human races, zero in on the differences from humans and spin culture from it, and again, make notes for your players so they know how to run with the differences. How does the long life of elves impact the culture of the elves in your campaign? Maybe elf hair keeps growing even after it is cut, so there is a ritual associated with burning cut hair.
A little more on cheat-sheets -
I'd make this a simple bullet list, divided into sections that start with "What You Know About..." or "What Your People Think About..." or "What Your People Say About..."
For example, consider a Shogun-era Japanese game:
"What Your People Think about Foreigners"
Real foreigners are knuckle-dragging barbarians, but since they do not know the proper way, give them some mercy.
Chinese, Koreans, Ainu are also barbarians who may make mistakes, but they should know the proper way of doing things.
As usual, Pathfinder to the rescue. Totally 3.5e compatible and with costs and stuff, not d20 Modern style.
Here are Pathfinder's firearm rules covering up through the revolver/rifle era, and here's the modern weapons info from their visit to WWI in Rasputin Must Die!, and here's the new technological weapons info from the new Technology Guide that has lasers and stuff. Enjoy.
The rules contain dials for adjusting price based on rarity (for example, the modern firearms list prices are 10% what they could be if you wanted to call them very rare). Tech items are priced just like equivalent magic items. The Craft (mechanical) skill is for high tech items and other gizmos, Craft (firearm) and the Gunsmithing feat for normal firearms. Craft DCs aren't in the SRD but are on page 101 of Ultimate Combat, and it's 20 for normal firearms (higher for siege firearms).
Best Answer
If you're really interested in making this an important part of campaign flavor, you might want to take a page from 4E Darksun and the "Reckless breakage" option:
For your world, substitute "human-made" for "nonmetal" and "dwarven" for "metal". I really enjoyed this in Darksun, since the breaking is the player's choice — it gives a bit of fun to what is normally an "aw crap, wasted turn" roll. And it doesn't make the default a penalty, since no one has to reroll — it's just an added option.
You could vary the "natural 5" to give different levels of dwarven quality, or even make it so dwarven weapons never break in this case.