I would go check out an apple store in a nearby mall or so if you have one first. All the apple employes will answer all your questions you have.
iMac's start out at $1099 for a 1.4ghz duo core processor and 21.5i display. Then they go up to a 27in display with a 3.4ghz quad core processor for $1999. Then they have about 3 other models in between those ranges. See all the models here.
http://store.apple.com/us/buy-mac/imac
If it was me, I would purchase a 15inch: MacBook Pro (laptop) to get all the wonderful features of portability, work on couch, work at kitchen table, work in garage, hotspot coffee shops etc. and also purchase a second 24"(plus) monitor to plug into the MacBook when I was at home doing work.
http://store.apple.com/us/buy-mac/macbook-pro
Don't know what your budget is but even though Mac's can be a little spendy they will last you 10+ years and hold up. Of you are using any of it for work or business I would then suggest write it off as a business expense. Another way to save a few bucks if you have any family members on school they can get a school discount.
http://store.apple.com/us-hed/buy-mac/imac
Enjoy