The problem is that Ancestry is using different markers or classification of markers as other places, so some of the results they tell you don't match well. I don't want to say they're wrong, because they are probably accurate representations of the sets of markers they choose, but you end up with a lot of very off results.
Getting your mom tested is totally the way to go for the next step. But if your goal is to find out more about your dad's side, I wouldn't use AncestryDNA. I would test at 23andme or FTDNA where you can get your raw data download. Then upload it to Gedmatch.com (it's free) and play around with their ancestry programs. You'll get a much better sense of what you're working with. And there are some programs that will let you compare your and your mom's results too.
AncestryDNA is a good place to be plugged into the database. You may find some family from your dad's side. But it's not the best choice for ethnicity tests. They say they'll be releasing raw data in 2013 (to existing testers too) and that it will be in a format that gedmatch can use, but we don't really know until we see it.
If your goal is to try to find some paternal relatives, then you will want to be in as many places as possible. You're already at AncestryDNA (and no need for your mom to be in that database except to weed out the relatives on her side (the ones that match you both)). I'd do 23andme then do a transfer of raw data to FTDNA (you can't go the other direction unfortunately). And I'd always do GedMatch, cause it's free and will do some matching as well as the ancestral stuff.