I bought the NRG Epac off ebay for about $70. It is supposed to make everything that uses power from your battery to work better including your spark. I did not notice any hp gains of course, but my headlights did get alot more brighter. It is also suppose to make your car start up faster and idle smoother. Mine does startup alittle better but not much.

did you notice a more steady idle? i did when i did a grounding kit

yeah

is this just sort of like a capacitor for your cars entire electrical system?

I believe it's more like an amplifer. It's really good to have if you have alot of load on the battery, because it takes some of the stress off. I've also heard it's better to get one of these instead of a distibuter cap or some other minor ignition upgrade.

being too lazy right to check myself,
it would probably be safe to assume it would work with an exernal coil set up?