Namibia is much, much safer than SA, if that's your comparison, mainly because it has managed to control illegal immigration to a greater degree and because there is not the same amount of poverty and unemployment, and Namibians are fundamentally honest.
Take the normal precautions and you will be fine. If you leave electronic equipment and cash unprotected you are asking for trouble but it might not happen. Plastic money is generally accepted in Windhoek and the other smaller towns (there are only a couple anyway!) but in rural areas it might be more of a problem.
If you get the chance, take a drive to Swakopmund and give it my love! There are two routes, the main tar road (B2 I think) via Usakos and Okandja, and the southern routes which are mostly gravel (unless tarred since I last did them) but usually in good condition. It's worth going out on one route and back on the other.
And one last thing, Windhoek Breweries produce the best beers in the world, and I hope you're not a vegetarian. Enjoy!