What do I need to wifi enable an existing printer? Or must I buy a
printer already wifi ready?

I recently purchased the Linksys Wireless server and it works well
with Windows XP, just plug it into your USB printer:
 	
The Linksys Wireless-G PrintServer lets you connect a USB printer
directly to your network, eliminating the need to dedicate a PC to
print sharing chores. Using a PrintServer frees up your ""print share
PC"" so you don't have to leave it on all the time. It also removes
the printing bottleneck, and sets your PC free to do more useful work.

http://www.linksys.com/