Just to keep your options open you can get a Mac Mini, a 21"+ monitor, and you can still get the wireless Apple keyboard and mouse for it. You would probably come out $200 ahead or so by doing this, depending on how expensive of a monitor you get, compared to an iMac.
You have to pick whatever is best for your situation and budget but I just wanted to throw that option out there. The Mini is VERY easy to upgrade.
Not only that but a lot less expensive to replace with a new one a couple years from now compared to having to buy the monitor over again. I view the iMac as a laptop, you are pretty much stuck with the initial purchase.