just a word of warning before spending on the wireless router: If you have a wireless network, hooked to the internet, and you want the DS to join that network, you either have to run with no security, WEP security enabled or a combination of MAC Filtering and WEP. Unfortunately none of these options are secure to the possible use of bandwidth by other people who have a small idea of what they are doing.
The WiFi USB thingo from Nintendo, on the other hand, will only let the DS & Wii connect to it, so these should be safe from the risks mentioned above.
You need to do your homework before you lay down the cash... are you in a risky area? Do you always have your pc on? How far away from where you usually play games is the PC? Do you have a Mac? etc etc etc