have you decided on a brand yet? if not, try the infinitys. curcuit city usually has them on sale or crutchfeild has them cheap too. if you got the money, boston acoustics are the best. i have bostons in the front and infinitys in the rear of my rodeo. they are only 4" but the bostons sound wayyy better.
The infinity's are a great choice especially if your a fan of bass. They produce the best and cleanest. If you ever need a speaker size for any vehcile you can get them at www.crutchfield.com. I also added a pair of tweaters (polk) on the front door and it really enhances the overall clarity.