and I only give predictions for these two because I can't predict what a console that's owned by toddlers and the elderly alike is going to do. No offense intended.
Microsoft: Sales will more or less remain stable. No spikes and no drops. Just regular numbers. Think about it, what game is going to get the 360 a major sales jump? There is none. Gears 2? Gears was great but if don't you think that people that want Gears 2 already have a 360? And if MS didn't dominate the market after Halo 3 then what else do they have? This is not the year the 360 dies; far from it. It will be the year we start to hear about Xbox 4.
Sony: Remember way back in the day when the PS2 got GTA III, FFX, and MGS 2? Yeah, that was the holy trinity for many. Well 08 may come close to that or even surpass it. Team Ico's game will be unveiled. GT5 will be released. God of War III may be out for the holiday season. Little Big Planet will change the way we platform. Home will change PSN. The Final Fantasy's may not appear but will be ever closer. White Knight Story is lurking. So is the Getaway. Resistance 2, Killzone 2, and Metal Gear Solid 4. Holy crap people. Now this is the boom that gamers have been waiting for. This is the year when PS3's start out selling 360 on a regular basis. I'm not saying that the PS3 will re-claim the USA, but it will make a major dent in MS' claim for second. That's the good, the bad is that after all of this if the sales don't increase then Sony will never get the USA back. The chances of the latter are, however, very slim.