The atom is designed to be super low power, its for small systems like nettops and netbooks, it doesnt give good performance but it does give great battery life.
The i3 is the lower end of their current processor lineup, they come with integrated graphics and are all dual cores with hyper threading as far as i know.
The i5 is a bit more confused, some of them have integrated graphics, some dont.
The i7s are all quad cores with hyper threading and require a separate graphics card.
Dont forget to check out AMD systems too, they work just as well and are sometimes quite a bit cheaper.
Which one is best for your business depends on what you use it for. Are you just using basic office apps then an i3 or an Athlon II x2 would be perfectly fine. If you are doing stuff like Maya or Adobe premier then you want the i7 or a Phenom II x6. The i5s and Athlon II x4 and Phenom II x3 and x4 are good if you cant go all out on an i7 but still need quite a bit of CPU power.