It really depends on what type of software you like and how closely you fit with the Nintendo target demographic. This time round Nintendo are targeting die-hard Nintendo fans and some of the more untraditional market segments. They have deliberately concentrated on interaction and originality rather than go head-to-head on technological capability with the other console giants.
You may not consider yourself a die-hard Nintendo fan, but if you already have a GameCube then that is added incentive, as all your Cube games will work on the Wii. Ergo, you can trade your Cube in or sell it to help offset the already cheap cost of the Wii, and without having to sacrifice all of the games you've already bought.
With regards to software, there are few really good titles available at the moment but 100+ more are being released between now and Christmas. I personally thought Zelda was worth paying $500 for just to play that game alone, but I'm a die-hard Nintendo fan so take that for what it's worth
If, on the other hand, you like online team-oriented action games (driving, first-person-shooters etc) then you really can't beat XBox live.
In summary: If you're a male in the 16-24 age bracket and you have broadband then you'll probably get a lot more fun out of XBox 360/XBox live. If not, then you'll probably do better to trade in your Cube for a Wii. If you like flushing large wads of cash down the toilet for a small range of really crap games and a blu-ray player that your current tv probably can't even support properly then buy a PS3.