ya, 802.11 is a standard 🙂
Provided you didn't do anything special (i.e. mac filtering, LEAP/RADIUS) with the existing network, either OS X will pop up a box asking you if you want to connect to the network, or most of the time you should be able to click on that little icon on the right hand side of the menubar that looks like a bunch of curved bars stacked on top of each other and click on the name of the network to connect.
However, the iMac you're getting supports 802.11g as well as 802.11a (well, not 100% sure about 802.11a support in OS X with the card in the iMac, but i am sure about g), so depending on what you're wanting to do and your internet connection (what if your wifi is slowing it down? 😱 😱 ) you might want to upgrade to a g router, because 11b's max is 11mbps theoretically and 11g goes up to 54mbps, and with proprietary stuff like speedbooster in some routers, can go up to 125mbps.