Definitely go with a good quality padding, something in the 6-8 lb. range. Think of the padding as a shock absorber. It takes some of the traffic and impact and helps extend the life of your carpet.
I'm not a huge fan of berber, though. Most berbers are made of olefin or an olefin/nylon blend, and olefin can be a pretty rough fiber. Olefin is a solution-dyed fiber (dyed while being made into the yarn instead of after the carpet is made) and repels stains easily. Nylons are strong and polyesters are very stain resistant, and both give wider ranges of color than olefin.
There is a new line from Mohawk called Smartstrand. The thing about this carpet is that it has a limited lifetime stain warranty. The stain protection is built into the fiber instead of just being applied during production. It also has a great soft feel and nice performance ratings. What impressed me most was hearing one of the salespeople from one of my sister stores (a guy I've known for years from when I worked as a sales rep for one of the carpet companies) rave about this carpet. This guy has always been strongly pro-nylon, but he's raving about a polyester. It's like Ronald McDonald raving about Wendy's hamburgers.
Also, check the fiber twist, density, and PAR (Performance Appearance Ratings) and make sure the carpet is good for where it's going. Low traffic rooms like bedrooms can get by with lower PARs, but you definitely want a good PAR and a good density on stairs and in family rooms and hallways.
SplshMtn99, it even got out the mustard and pet accidents? Those are strong staining agents and usually among the exclusions in the manufacturer's warranties.