Emission, I must back up the people recommending OEM plugs. They are pretty cheap and very effective. I have yet to find anyone who has significantly improved anything by installing anything other than OEM, but I have heard (right here, in fact) many stories of degrading performance and going back to OEM plugs. You don't mention if your car is turbo-charged. If it is not, then the OEM plugs are kind of funky (see picture). I think they are that way for a reason. I got a brand-new set of Volvo-branded plugs from volvoparts.com for something like $7.00. With that price, I felt no need to experiment, since Volvo has already put in some work into choosing the right plug and found a way to make it very cheap. After I put in the new plugs, not mucha has changed, nor did I expected it to. As you see from the pictutre, my original plugs after 27K miles look like they can easily go much longer, but I changed them out anyway since I already had the replacements and they were already out. Next time I don't think I'll be in such a rush and wait until the plugs have at least 35K-40K miles.

Good luck.