Contacting the main stream sites will be a waste of your time and theirs unless you have some samples of your writing already up on the internet. They will probably not look at you unless you have worked in writing already at smaller sites. You need to go to any sites that are looking for people to write on a volunteer basis and get a few hundred reviews under your belt.
I have known a few guys who have gone onto sites like IGN but you have to put your time in at small sites to get some experience. That is how it works around here for writing, get the experience and then go for the big jobs.