it will vary from company to company. i do a lot of recruiting and usually skim thru resumes sent in via the web very quickly. you get 100's for openings and most are totally mismatched to the role. it's basically far to easy for people to apply for multiple jobs so they just spam their resumes out. you need to differentiate yourself.
if this is the job for you, and you are a good fit for the job i really recommend you send something directly to someone at the company. if you know the hiring manager go straight there. if you don't, see if you can find the head of recruiting, or head of HR. call their main number and ask for the name of the head of recruiting if you can't find it. then you can probably work out what their email ID is likely to be based on what the email is for other people.
send a cover letter saying that you applied for the job online but think it is such a great fit you wanted to follow up directly, and obviously attach you resume.
do not assume that they are painstakingly reviewing every resume. they will do this quickly and if you are just one of 500 web applicants you will likely be skipped.
good luck!