If you purchase a new notebook right now, choose the model with build in wireless 802.11b/g (or WiFi) connectivity. The rest you have to buy a wireless router only (around HK$300 to HK$500, depends on model and brand name). The function of the wireless router is to "split" the internet connection to your desktop PC by wire and share with your notebook via wireless.
Meanwhile, if you want your desktop PC to be connected with wireless, you may have to buy one USB wireless client for it. However, it is an option but not a must.
You can ask for opinion with the sales when you purchase the notebook.