I figure I'll be going with the Nintendo Revolution come 2006.  Essentially it boils down to a few solid reasons why:
 
1 - it'll be the most affordable
2 - it has a crazy new controller that I just have to have (imagine dueling someone online with that thing!)
3 - it is guaranteed to have the next Zelda, Smash Bros., and Metroid Prime, all of which I can't live without.

Beyond that there are some other perks, such as brand spanking new graphics, free wi-fi out of the box, and downloadable classics that only sweeten the deal.  I suppose the competition isn't all bad either though...the XBox 360 could find its way into my collection if Halo 3 becomes a huge hit like it's predecessors, and if the PS3 has a significant advantage in visuals plus some kicking RPG's, then I might be sold on that too, assuming I get an HDTV and an extra 600 dollars that I'm not using.  I am hoping, however, that Nintendo's new system is so perfect in every way that I won't need to buy any other system, because I am SO sold on that Revolution.