Any card, not just credit, Solo/Switch/Maestro/VISA Electron/otherwise
The adapter from GameSeek cost me £26.49 (I think!), where it retails for £29.99
Also, just had a quick blast online (While i was at uni, f**ked up one computers internet connection... read the instruction manual before you go ahead and plug the USB adapter in! Install the CD software before applying the USB Adpater!

). Took a while but eventually got a 4-player game in, some guy disconnected (Probabily cos he kept losing

), and after one grand prix (Where i came second... first time though so come on, props eh? Especially when i hadn't played any of the courses chosen!) another d/c'd, and then there were two... ended in a tie dammit! XD
Pass on your Friend codes so i can add you, and then be online tommorrow, or at a chosen time (Not the weekend plz!)
P.S. ONLINE RULES!!
[EDIT] Nintendo Wi-Fi Friends Code!