I have FIOS, with a verizon 9100 Series 3 router. What wireless card should I get for the Presario (Compaq) running XP?

Any wireless card or USB adapter will work.

There are plenty of choices for wireless USB sticks. Foe example: Newegg.com - NETGEAR WG111US IEEE 802.11b/g USB 2.0 Wireless Adapter Up to 54Mbps Wireless Data Rates Hardware-based 40/64-bit & 128-bit WEP encryption

The advantage of getting a usb stick rather then a card is you can plug it into anything. Very easy to do. If you install a card (wireless) then it is dedicated to just that one machine. So if you ever buy an older laptop you can use it in that also.