Check out some of the performance auto sites. I can specifically recommend www.thirdgen.org It is primarily a 3rd gen Camaro/Pontiac site, but it has LOTS of info on performance chips.
General concensus is there is not a lot to be gained on a STOCK motor with a chip. As several posters correctly pointed out, most aftermarket "performance" chips just bump timing slightly.
If you compare the actual programming of an aftermarket "performance" chip to a stock chip you will not see a huge difference.
Now if you are talking about a modified motor. i.e. freer flowing exhaust, bigger cam, ported heads, ported manifold, bigger injectors or some combo thereof an aftermarket chip can make a huge difference.
The catch is that not just any aftermarket chip will do. It has to be custom burned to your setup and tuned by reading O2 levels and measuring performance.
I have recently been learning how to program and burn chips for my TPI Chevy. There are big gains to be had, but they do not come easy, nor do they come with just slapping in an aftermarket chip.
If you are interested in burning chips, check out www.tunercat.com or www.moates.net
Sorry if this wasn't what you wanted to hear, but I hope the info/links are at least useful.
-Z
edit: spell