Well Nintendo has been pretty vocal in that if they stop making systems, they'll close up shop completely. Anyway, here's my take based on limited info:
Sony will be successful with the Playstation III, but I don't think it's going to be a squash match either. I think most people will get one eventually, as a second or even third option, as the casual masses will likely have to wait for a price drop. Plus with the cheaper model, you won't have the full on resolutions to take advantage of Blu Ray, so I don't see many outside of the desperate to own one buying it. The games, while good, aren't much different from the current gen, except with better graphics. While the eye candy wow factor is there, I don't think it's enough to get more than hardcore types initially, and even many of them may buy the two competitors, or even apply that $600 toward upgrading, or purchasing a new computer. The arrogant comments from execs don't help, but I don't see it taking a ton away from the PSIII. It's just not going to get enough early adopters IMHO, which narrows it's lead a great, great deal.
Microsofts, 360 has surprised me. I'm still going to hold off awhile longer, but Gears of War, Chromehounds, and others are really impressive upcoming games that a lot of action gamers are going to want. Graphically speaking the 360 holds it's own against what the PS3 titles look like, and the Online Live serviceis as painless as ever. The cross platform integration with PC gamers running Vista Premium also seems intriguing. The Keyboard, Mouse Vs. Gamepad wars will finally be decided (The KB M is going to dominate on the FPS side of things) Plus you can score some used launch titles cheaply these days, so you can start off a little more affordably than before though the $400 might even be too high for some folks.
The Nintendo Wii looks like an awesome value proposition, and honestly I think as long as more captivating different games come out, and are marketed right, Non Gamers will probably really get into it. It looks so far that enough third party franchises may hit the machine to get casual fans to give it an honest look, and that same alternative gameplay is going to attract a lot of the hardcore too. The FPS gameplay may even be a serious contender to the Keyboard Mouse combo, so I wouldn't be surprised to see some PC gamers pick it up for some of the exclusive Shooters. I think some people will be turned off from the visuals at first, but if enough games come through on their promises of fun gameplay, Nintendo has an honest shot at a #1 spot. That said I think the PS/Xbox names really gained ground over the last two generations, so that may prove to be a steep uphill battle.
Computers also look to have a pretty good run alongside the consoles. And you can even count APPLE in the fight thanks to BOOT CAMP. Of course it remains to be seen how well those might run Vista, but Microsoft's next OS looks VERY Gamercentric. And not just for the Hardcore FPS, or MMO gamer.
The OS will have plug, and play installations. You'll be able to pop in your game, start playing right away, and the files will install in the background.
It's going to have all of the info on your games, the ratings, and parents can even password lock the OS from allowing M or AO titles from running/installing! On top of that Microsoft is bringing a Windows based version of Xbox live to the OS, and have hinted that some games may be cross platform compatible! Imagine playing a shooter on the PC against your Xbox 360 toting friend! To top it off DirectX10 looks to blow away anything the consoles are doing. (Check out CRYSIS from Crytek to see what I mean)
That said casual types will probably still get a console for gaming, but it certainly could attract some into making their computer thier prefered game system.
Having said all of that, I think it's too close to call. If I had to nod it to anyone I'd say maybe Nintendo by a very small margin, as I can see most folks even buying that as a second system, followed by Sony since by year 4 or 5 they can get more affordable, and some will be picking that up as system two or even three, Then the Xbox360 RIGHT behind it. I'm thinking only a few thousand machines between the three. You can't measure the Computers since they're bought for so many different reasons, but I think you'll see a spike in PC games once Vista gets itself settled in.