OK... sparks from the negative batt. terminal?!
 

Here's the problem:
Everything was going fine while driving today; no issues. I got home from work, started working on wiring my amp to the four speakers. The back seat is gone, and I screwed a ground wire into the driver's side rear seat bracket on the floor, and attached all the corresponding speaker wires. The ground wire isn't attached to anything yet. My next step was to try to put an accessory terminal on the positive sidepost... one of those flat things that just goes around the bolt to attach a wire to. So I started taking off the negative terminal, and as I remove it, sparks jump from the wire to the terminal; as if I was removing the positive wire first. I'm almost certain that this shouldn't happen. I moved on and took off the positive terminal to attach the acc terminal.
Eventually, I gave up on the crappy acc. terminal and reattached the positive cable. OK, all is well. I go to attach the negative terminal and more sparks ensue. OK, very weird. The lights still turn on in the car, so I don't worry about it.
I came back outside a little later and try to start the car. All the lights come on, great. I try to start the car, it turns for half a second and then everything dies. No power, no nothing. @#$%!
I pop the hood, remove the negative terminal (no spark this time), remove the positive terminal and reattach it to make sure I didn't put it in loose or something, go to reattach the negative terminal and BRZAP, more sparks. What the hell? I didn't bother trying to start again. Anyone know what's going on?!

You have a loose connection somewhere. Probably at the acc terminal.

You got sparks from the negative terminal because you were completing an active circuit (the lights came on) The sparks are showing a current draw. There shouldn't be big sparks but there can still be sparks. Big sparks are telling you that you are hooking a hot wire directly to a ground. Check if there is any power on the wire when it's disconnected.

Theoretically you can remove either terminal first. Negative is always recommended. If the wrench/socket/plyers etc accidently touches chassis ground you won't fry yourself. If the positive is disconnected first you can complete the circuit to ground with the tools.

Try disconnecting the amp and try again. Maybe something you did is drawing down the battery.

OK, looks like it was a loose pos terminal on the battery. I don't know why that would cause it to die when trying to start the car and not when the lights are on, but I tightened it up and everything's fine now. It's always the stupid little things, isn't it? :rolleyes:
As far as the sparks, I guess I just never noticed them before, seeing as it's dark out now. I was under the impression that there are never sparks when removing the negative first, but actually thinking about it seems to make sense that there should be. Thanks for the help (Ben, too :D)