I had some hand numbness on my previous bike - a Nashbar tourer (from 1992 - not the new one.) I did a couple of tweaks that helped a lot. I raised the bars so they were almost level with my saddle, and I rotated the bars so that there was a flat platform on top, flowing right into the brake hoods - much more comfortable. My numbness went away.

On my new tourer, an LHT, and on my "fast bike" - a Specialized Allez - I kept the same arrangement. The flat place on the bars behind the hoods, flowing into the hoods, makes the biggest difference. I also found a stem that raises the bars so that their almost as high as my saddle. Again, no hand numbness on either bike. I also put some gel pads under the bar tape on top. I think it helps.

I have a 29er I bought for a tour on the Great Divide last summer. Since it was for a longer tour I wanted to be as comfortable as possible for day-after-day, long rides. I put Ergon barends on the flat mountain bike bars. I bought the biggest ones. They were VERY comfortable. The only thing I don't like about them is that the barends are attached to the grips so you can't rotate them independently. There is a large, flat, section of the grip that bulges out to provide a really nice place for your palms. However, if I rotated them so they felt perfect for my palms, I also rotated the barends to where I didn't want them. I had to settle on a compromise between the two. It would be nice if you could set the angles independently.