Depending on how much money you want to spend, 2, 3, and 4-way wont make as big a difference. I've heard nice two ways sounds worlds better than any 3 or 4-way. The MAIN thing you need to be concerned with is the RMS power rating. This is the amount of power a speaker can handle over an extended period of time. Most aftermarket radios put out between 18-25 watts of RMS power per channel. So with that said, the closer you can get to that number, the better off you will be in a lot of cases. If you can, try to pick out a pair of speakers that have a rubber surround, they tend to last longer than cloth or foam. Rubber tends to last considerably longer in a rear deck mounted speaker.

Sony has never been one of my favorite brands. I've heard their Mobile ES line is pretty good. But never the less, Im not a fan of 99% of what Sony has to offer car audio wise. Pioneer, Alpine, Kenwood, and Fosgate are all good brands that are affordable at the same time.

Try stopping by the Audio and Electronics section of the forums. Theres a lot of good resources there along with quite a few people that are very knowledgeable.