Bose in 2002 Sierra Denali

You'll end up paying a LOT of money to turn the stock system into a bose equivalent.  And it will never sound anywhere near as good as aftermarket speakers, especially focals!

The bose system consists of amplifiers as well as speakers, and they are designed to work only with each other.  Thus if you got the bose amps you'd need to get new bose speakers all around as well.  And if you tried to replace the standard stock speakers you have with a pair of bose speakers it would probably fry the head unit (bose uses 1 ohm speakers which would end up drawing too much power from the head unit and causing it to overheat).

Definitely, absolutely, without any sort of doubt, install those focals!!!  You will be very pleased with the sound quality.

Absolutely--upgrading your speakers will usually give you the biggest increase in overall sound quality when compared to your all stock system.  Of course the next logical step would be to upgrade the head unit or get external amps for the speakers, because that's where aftermarket speakers really sound awesome.  But you can get by with just using the stock head unit for now, though I don't know if I'd get focals...

don't get me wrong, Focal makes some of the best speakers out there.  I just don't think they'd sound any better as long as they're still being powered by the stock head unit--at least not enough for you to be able to tell the difference.  Now if you do intend to add amps later on down the road then I would say the focals might be a better choice for you as eventually you would be able to tell the difference between them and a less expensive brand like pioneer or jbl.

However, if all you're trying to do is replace a pair of blown stock speakers with aftermarket ones, I would go for something less expensive than focal.  Just my two cents though.