Welcome to WiiChat Zayin.
The only pure singing game that I've played is American Idol Encore 2. It was good fun but it didn't allow for any downloadable songs.
As far as I'm aware the Wii version of Karaoke Revolution doesn't have downloadable content or online play at all either.
Guitar Hero 5 or Band Hero (depending on what kind of music you like) could be better choices if you want a wider range of songs. The good thing about GH5/Band Hero is that you can use any combination of instruments that you want, so you could have 4 people singing at once if you wanted and not a single guitar in sight, where as most singing games only allow for 2 players. They also feature downloadable songs and online play, so they should last a lot longer than the average singing game. You can buy the games seperately and then pick up a couple of the usb microphones for about $10 a piece.
Hopefully Karaoke Joysound might find its way over here one day. It is a downloadable WiiWare game in Japan that they can download almost any song for. Licensing problems may stop it from ever seeing a western release though.