Maybe you want to start with your end goal and work backwards. You want a low gear under 25 gear inches and a high gear that is around 90+ gear inches, though that is less critical. And you want your shift transitions to be reasonable, around 15% is a good goal. A classic combo is the XD above and an 11-32 or 11-34 cassette but many other combos work, some cheap, some expensive. Try working some combos here and don't forget to enter your wheel size and your desired crank length. For most people 172.5 is the preferred choice but you can go 170 if your are shorter than average and 175 if you are taller or do a lot of climbing. http://www.sheldonbrown.com/gears/

Keep in mind that it is recommended to avoid spending long periods on the extreme chain line angles = biggest chain ring and smallest cog and vice versa because of wear on the chain and system is greatly increased.

It depends on what you are looking for price wise and what you already have on the bike. Also remember you can change the chain rings on some crank models. Consult with your LBS or friendly local mechanic for that.