Yes - you need a wireless access point for your modem and a wireless gaming adaptor for the X-box. My son and I did this for a similar setup - his PC/modem downstairs, X-box/TV upstairs. He used a Netgear combined ADSL modem/wireless access point/4port router (DG834G - 802.11b & g) at the PC and Netgear gaming adaptor (WGE111) onto the X-box. Its just simpler to set up the combined modem/router/access point rather than separate units. I prefer to use wireless hardware from one manufacturer as it should be compatible! Hardware would be different at the PC if you have cable broadband and/or just want to add a wireless access point to your existing PC/modem.
Hope this helps
MikeD