I was wondering if anyone could tell me what premium (platinum/double platinum) and super-premium (iridium) spark plugs do for perfomance. Do these provide significant imporvements performance-wise and is it worth it to spend the extra cash. Also which brands are best. Is Autolite a good brand?

The only difference is the tip of the spark plug. There are different theories on what is better. I have had people tell me that plain copper spark plugs were ideal, because tehy have better spark and "just changing the spark plugs to copper gained me 40whp on my modded subaru"

Platinum is good for people that want a more consistent spark, it isn't necessarily better, it just lasts longer because the tip doesn't break down and get as much carbon buildup on it quite as fast. They are supposed to last 50k to 100k miles and most people i've encountered dont' change those plugs for 100 or much more.

Iridium is the same deal. better tip for cleaner spark, without heating up the metal as much, and yadda yadda. Not sure on mileage, but i know my car came with them from the factory and they don't have to be changed for 100k miles.

the ONLY brand imho to go with is NGK, no matter what kind of car you have. If you dont' care for them, then ONLY put what you took out back in. If you are shopping at wal-mart for autolite plugs you officially lose. Go to a real parts store, and pick up some NGK plugs or the OE plugs.
For reference:
General motors vehicles: ACdelco spark plugs
Japanese imports: NGK spark plugs
Fords: Motorcraft/ older used Autolite
Dodges: Champion (eeeeeekkk!!) ? unsure on if they changed that recently
German cars: Bosch (mixed feelings)

+1 on the NGKs i love em. ive used iridiums and platinums both seem very effficient

I like NGK and Denso. If you're tuning a car go with copper. If you're stock go with iridium. Also, avoid Bosch Platinums if you have a turbo-charged car. They will foul in no time at all.

NGK's

NGK IMO.

Here is a comparison of different spark plugs in an SRT4 on a dyno. It gives some good information on different plugs and how consistant they are. The NGK iridiums are the most consistant out of them all.

http://www.sparkplugs.com/sparkplug411.asp?kw=SRT%2D4+Dyno+Results&mfid=0&KID=3095

Here are the results from that link.
http://www.sparkplugs.com/glossaryImages/499.jpg