Netgear wgr614/belkin/dlink 524 or 624 wireless routers are available for 30-40$ after rebates at CC or BB. Ideally you could also get a router with a print server but that would cost more, for now leave the printer connected to the desktop and share it.
Routers provide a basic firewall protection by their NAT (internal routing), some also have SPI feature (wgr614), but these are not comparable to hardware firewalls from Sonicwall, Cisco etc which are feature rich devices meant for larger networks. In addition, you can download Zone Alarm for free and that much better that SP2's built in firewall. A combination of a router with a software firewall like ZA is more than adequate for home use.