No, you're right, the Wii seems like a good choice. I was really wanting to get one (if I saw them), but with a baby on the way we just can't afford to drop $250 (plus games/etc.) on a new system right now. Maybe if I quit this collecting disease, it would help 
. That's the main reason I was leaning more towards some of the older systems, $100-$130 is easier to stomach for a new system. I'm not unhappy with the Gamecube necessarily, but being a person who can only afford to have one new system at a time, I started to find that it didn't carry a lot of the games you could get on all the other systems (including handhelds). That, and the fact that they aren't even making games for it anymore makes me lean towards a new system. Even with the success of the Wii so far, I guess that there is a small part of me gunshy with Nintendo again after seeing that they seem to have a lot less variety/longevity with their games/systems than the other two big names do.
I went with a 'cube at the time for the price, and the fact that I just wanted a "fun" system, reminding me of the NES of my youth. I still think that's the most fun I've ever had with a game system, the old 8-bit NES (I need to get mine fixed). Anyways, that's why I was leaning towards a Wii - and you're right, that probably would make the most sense for someone like me. Sometimes I just feel like I "backed the wrong horse" with the Gamecube last time, with the PS2 and Xbox seemingly being the better systems in the long run. Its all subject to what people like though I guess.
As far as the last generation (2000-2006) goes, yeah, if you only went with one console, and that console was the 'Cube, yeah, you probably picked the wrong horse. Still though--there was a bunch of good exclusive games for the 'Cube that you couldn't play on any of the others. It was an alright little system.
The PS2 is a different story. It's the oldest of all the systems out there, but it still sells a decent amount, and it's still being supported with new games, to some extent. God of War II game out a few months back, Guitar Hero: Eighties is coming out this month, and Guitar Hero III is coming out later this year. I'm sure there are many more--those are just the ones I'm interested in. But still, if you buy a PS2 now, that's still nearly a seven year-old system. Who knows how long it's going to be supported.
But whatever you do, don't get an original XBox--it's just as dead a system as the GameCube is.
I'd still say a Wii would be the best bet for you. It's a brand-new system, so it's going to be supported pretty well for at least a few years. It's fully-backwards compatible with all your old GameCube games. You can hook it up online and download many of the old NES and SNES games you enjoyed back in the day (at a nominal price, of course). At $250, it's the cheapest of the next-generation systems, and it comes with a game, so you don't really have to buy anything else, except maybe a second controller. A lot of people are happy with the pack-in game, Wii Sports. And even though it's been out for a year less than the 360, the Wii is on the verge of overtaking it in terms of total sales (around 10 million or so), so you can be sure that it'll have plenty of support and new games for the forseeable future.
As far as the cost goes, since the Wii is fully backwards-compatible with the GameCube, you could go sell or trade your GameCube in, along with any old GameCube games that you don't want to play anymore, and that would knock a chunk off the price of the Wii.
