I just bough some MB Quart 6x9's today and love them. They sound so much more clearer than my old Pioneers and they are only being powered off my old Sony head unit. Overall, I'm extremely happy with the purchase.
As long as your original speaker is a 6.5", any 6.5" speaker will fit in that space (just make sure they fit depth-wise") You might have to drill some holes in the speaker harness to secure the speakers, but its not very hard.