3 setup questions

[Don Ganso]

BACKGROUND:
I currently drive a '93 Saturn SW2, (which I will probably need to replace soon, but more on that later), and use a single 10" sub, (Cerwin Vega VMAX: 500 watts RMS), in a ported enclosure tuned to 35ish Hz powered by a 500 watt RMS (at 2 ohms) amp, (don't ask what brand, its embarrassing). After the answers I recieved to a question I asked in a previous thread, I have decided to purchase a Fi BL. I want my setup to have great SQ and plenty of volume.

Question #1:
Is the 18 inch sub going to be amazingly louder than the 15 and what options would be best to improve the SQ and volume of the sub?

Question #2:
If I am unable to upgrade my amplifier immediately, (budget constraints), will underpowering the sub destroy it? I have heard that I could get away with it as long as I keep the volume low until I get a better amp; is this true?

Question #3:
Eventually, I'm going to need a new ride, but i imagine that I will get the new sub and amp before then. Obviously, the stock electrical system is not going to handle 1000 watts RMS, so what can I do to allow my little Saturn to handle that power? I was hoping that if I did the big 3, I might be fine, because I really don't want to put money into things that I cannot remove from this car and the big 3 seems to be the cheapest upgrade. Will it be enough, or will I need a new alternator?


Thank you all for any input you provide.