Battery or Altenator?

[68Formula]

About a month ago, I drove my Pontiac and left the lights on. Jumped the car all was fine.

Drove the car yesterday. Stopped and restarted once with no problems. But it died at the gas station, and didn't have enough juice.

Went to jump it (it was dark by this time) both positive and negative are black and the routing is weird (long story). Before I started the jump vehicle, I momentarily swapped positive and negative. Corrected it, restarted the car and drove home.

Interior left blinker light stayed solid when signaling left, and nothing signaling right. Gages were completely dark. Wife followed and said the brake lights were fine, but the taillights were so dim she wasn't sure if they were on (they were). My headlights were also so dim (but on), I couldn't see much of anything (good thing I was only 1 mile from home and most of that was through our subdivision).

Put a voltage meter in the cig lighter, and after rejumping it in the driveway it read 12.5volts. After letting it run, it was up to 12.8volts. Disconnected the jump car, and voltage dropped to 11.8v. When I put on the headlights it dropped to 11.2v. Turning them off, it went back up to 11.8v again. Turn off car and won't restart.

Is this a battery or alternator problem, or both (did I fry the alt)? Battery is heavy duty (biggest one that would fit - figured I need it since the car is not driven daily), but probably about 7 years old now. Alternator was purchased new in 2004 when I went to single wire.

Also, if I have to replace the alternator, I'd like to get one with more output since the future calls for fuel injection, electric fans, and power seats. What would be a good replacement (you know how modern parts stores are - you have to ask by vehicle and engine size). Something from a luxury GM car that would have lots of bells and whistles like a Cadillac, Olds or Buick? I don't want to spend a ton of cash on a fancy chome model, or high output aftermarket one, if I can simply buy one for another model that puts out more and costs about the same as a replacement. Again, it would have to be a single wire one.

Thanks.