Pretty much any 6.5 will fit, but you may find that you have to be a little 'creative' when it comes to attaching them to the sound bar. I (briefly) had a set of Sony's in mine, but am currently running a cheap component set made by Pyle. There is also a direct replacement made by Kicker that a lot of people seem to like. As for upgrading, you're only going to hear minimal improvement unless you replace the radio as well (IMO, of course).
No matter what speaker you choose, to get the most out of the sound bar location, take the opportunity to stuff it full of poly-fil while swapping the speakers. It'll help fill things out a little.