A few thoughts:
In a purely 70s-based system (i.e. turntable, or source, receiver, and speakers), I would say that any of the devices where energy is converted from one form to another is the most important element. So, that would mean the cartridge (converting the potential kinetic energy on the vinyl into electrical energy) and, on the other end, converting that electrical energy back into kinetic energy (the speakers).
While the other elements may affect the sound, their overall level of impact will be minimal when compared to the cart and the speakers.
As far as your blu ray question goes, well, that depends. Does your receiver support HDMI 1.3? If not, does the PS3 have at least 5.1 (or, better yet, 7.1) analog outs that you can run to your analog ins?
As I understand it, to get the full, uncompressed audio over HDMI from Blu Ray, you need to do everything through HDMI 1.3. So, for me, for instance, my receiver uses HDMI 1.2a, and, therefore, if I were to go with a Blu Ray player, I'd need to use the analog outs on the player and run them into the analog ins on my receiver.