Speaker wires... can't really help you there if the house was pre-wired. You could try connecting some thicker-gauge in-wall wire to the existing wires and pulling it through, at least for the main right and left speakers.
Sounds like what you want is just a new 7.1-channel receiver. There are tons. What other features are you looking for? What kind of devices are you using? DVD player, cable box, etc. How are they connected currently? This will affect what you'll need.
As for louder, I think you'll just have to crank the volume. I wouldn't worry about the wires. And a new receiver, unless you go crazy, won't be that much louder than your current one.