Well, I do this every year 'cause I'm a nerd. But I'm pretty busy since finals start tomorrow, so I'll keep it short:
Nintendo: Strong showing, but won't win. The DS system will be unveiled, it'll have wireless Bluetooth multiplayer and a few promising games, and it'll release in time for Christmas but won't be as huge as the GBA. Oh, and did someone say "New Miyamoto Zelda game?" The teaser is 100% incredible, go find it. Gamecube has a promising set of games for this year (Pikmin 2, Paper Mario 2, etc.) so they'll finish strong once more and make everyone who put $99 into their Gamecube feel like it was the best investment they ever made.
Sony: Will amaze the world with the PSP and jaws will drop with some really outrageous claims of what it'll be able to do. Don't forget, by now we were all supposed to have PS2s in the living room, downloading movies and games. When the system comes out, it'll be good competition for the NDS but nothing earth-shattering. Maybe we'll see more of Metal Gear Solid 3 or a surprise Rockstar announcement, but beyond that my expectations are low for Sony.
Microsoft: Will win the show by not surprising anyone. They'll have Xbox ports of the major PC releases coming (Doom3 and Half-Life 2), a few Xbox Live upgrades (including support for EA games), and some really good titles for Live that'll be out by Christmas. People will catch onto the buzz, and Xboxes will sell by the dozens on the appeal of Xbox Live and the release date for Halo 2. This will be the year Microsoft wins this battle of the console wars.
PC gaming: Will do the same thing it does every year: show about 3 triple-A titles and nobody will care about the rest. Doom 3, Half-Life 2, and one more might steal the show, perhaps in the form of another Valve announcement or a new Blizzard game (not likely).