To sum up a few things:

The basic theory of acheiving more HP is simple. The more air you can get into and out of the car, the better. An aftermarket intake is larger and less restrictive than a stock one. So more air will be taken in. Because more air is taken in, more fuel will be added for the proper mixture. So yes, an aftermarket Intake will increase your power. By how much? hard to say. It normally varies from 3-7hp depending upon the set up. (SRI/CAI)

My main problem is the price difference between intakes. A lot of people get AEM or INJEN intakes ($150 or more) which is good, but I don't see a problem with purchasing a $50 Ebay intake and swaping in a K&N filter. Which is what I've done.

As far as getting a SRI or CAI, it's up to the person. If you get a CAI, just be carefull in deep puddles. But regardless, you should always be careful in deep puddles. My judgement of a deep puddle is water deep enough to cover a stock tire and touch the rim.
A SRI is pretty good also, as long as it's away from any type of direct heat. As long as it's away from the Header (headers for you V6 guys) and radiator fans your should be ok
Personally, what I did was purchase a SRI intake (for $40) and put on a K&N filter. It's perfect cause on the 6th gen 4cyl, the filter is right on the side between the battery and the fender. Then for the summer I purchased a 90 degree Greddy turbo pipe and when the summertime comes around, I connect that pipe to my intake pipe and connect the filter to the end, and now I have a CAI. When it gets gold, I remove the secondary turbo pipe and go back to my SRI setup. In case you're wondering why a turbo pipe, they repel heat the best.

Here's my SRI setup



And here's my CAI setup




Of course this pic is before I got my new Valve cover