If you use a baffle, MDF or otherwise you can fit 6.5's in your door panels.
If you use a baffle you can fit 6.5's in most door panels.
Thus 6.5's will be easily transferrable to your next car with little to no modification of your current speaker holes.
Don't listen to Sarthos, he dropped a bunch of bull on you about the speaker size affecting volume and midrange performance. What really affects volume and performance and any frequency is your install quality. Go read a quick DIY about sealing/deadening your door panels, it really doesnt take much know how or expertise, you've just gotta be methodical and able to follow instructions. If you can do that you'll see a world of difference in speaker performance, as compared to the difference in 5x7's to 6x9's.
It's 90% install, 10% equipment.
You don't sound like you're too serious about this anyways, if thats the case, you should just get a decent set of 5x7 co-axials, just because everyone on this forum is in love with 6.5 component sets doesnt mean they are the thing for you, you'll be perectly happy with a set of 5x7 coaxials and a mid end alpine head unit.
However if you feel the need to make a serious musical investment a 6.5 component set up front with some 5x7's in the back running off of an amplifier with a 10 or 12 inch sub in the back would be a good starting point for a basic system.