I'll assume you're talking about the AEM kit:
A set of pulleys reduces the parasitic drag that the stock pulleys give you. They are made of 6061 alum. and are slightly larger than the stock pulleys to slightly underdrive your factory accessories. They will gain you a few ponies and increase throttle response. Most of the power gained is in the 4k range I believe, which is nice to get you going.
The AEM set will go for anywhere from $130--> $180ish. Try
www.groupbuycenter.com for a good price.
There is a downside. They underdrive your alternator, which means that it will output less power, but this is ONLY noticable at idle. If you have a large system, I'd stay away from these, or at least add a capacitor. My lights dim at idle, but its no big deal to me.
If you have any other questions, just ask away.
