Probably the most impressive progress of all teams in 2007 was made at the Hinwil-based BMW-Sauber squad. Because the current car has been performing so well, they're actually ahead of schedule with development and have switched focus to the 2008 car entirely now. This promises even better things for the next season and I wouldn't be surprised if they'll be able to fight for wins on a regular basis. With McLaren in hot water and a bit crippled, BMW-Sauber is bound to take over second spot from them (based on performance and not just by FIA decision). Will they be able to challenge Ferrari?
Um... based on what performance? McLaren is league above BMW in this season.
However, since McLaren will not use any Ferrari's intellectual property next year, I'm sure BMW will be able to challenge Ferrari; McLaren should be a bit above both of their reach, at least until Max Torquemada Mosley pulls another devil hunt from his loaded sleeves.

Jokes aside, team that gains technological advance is likely to keep it for longer than 1 season; at least that is what we had in the last decade or two. From that point of view, I would expect McLaren to have strong show in the next season performance wise... Probably Ferrari as well. BMW, I do see stronger than this season, but still not strong enough - it is not like everyone else will just sit while BMW push their limits. I'm not sure about Renault, hoping they will improve as well but would not be surprised if they continue with decline.
Of course, that is just from technology point of view. What happens with drivers and managers will play part as well. Just as example, there were rumours that R. Brown is considered - and contacted by McLaren on that matter - as replacement for Ron Dennis (yeah I know it is hard to imagine but one never knows). One way or another, I'd like to see 4 strong teams competing equally, but I'm not expecting to see that for 2008... maybe in 2009.
You can have it in any colour - as long as it's black. Or red. Or blue. Or green. Or white. Or navy blue. Or...