also keep in mind that different brands will have different mounts. i replaced my fronts with same-size kenwoods (6½) but only 2 screws would fit. for the rear, i went up a size from stock to 5x7 (i think stock are smaller, they look smaller) rockfords, and again, only a couple screws fit in. the rears are much easier to mod-mount though, plastic is really easy to drill through.
in both cases, the sound is exponentially better than the stockers. i´m weary about putting the rears into the hatch cover though, i think my sub would distort them more than having them in the stock placement.
good luck with whatever ya go for though! :-]