The laptop will connect to the router by wireless.  The current desktops will connect to the new router exactly like they connect to the old router.  When you switch out the router, make sure you do a complete network reset.  That is, turn off everything including the cable modem, attach the new router to the cable modem and to the two computers, then turn on the modem, wait about 5 minutes for the modem to synchronize with the network, turn on the router, then turn on the computers.  Then follow the instructions that come with the new router to set up the wireless connection with your laptop. 

Steve 

Your wireless router will replace the current wired router that is now sharing the internet connection.  The wireless router will have 4 wired ports and will broadcast a wireless signal that will allow the new laptop to connect to the internet in exactly the same way that it allows the wired computers to connect.  Thus, you will not have to change your ISP.  Again, just replace the current wired router with the wireless router.

If you want to access the internet from work across the street, this may or may not be possible.  Wireless routers have a range, under ideal conditions of 200 to 300 feet.  Ideal conditions means that there are no intervening walls, cielings, trees, cars, etc. between the router and the wireless card.  Real world conditions can have all of these things, and thus whether or not your router will have a range that extends to your work, cannot be determined without trying it.

Steve