Caching impression counts to avoid mysql inserts

We currently get a lot of mySQL inserts/updates on duplicate daily (millions) and we're looking at speeding up the application by using some form of counter with memcached and then clearing it on intervals with a cron. Is this possible? What happens if 2 people hit at the exact same millisecond? Will the counter still go up twice or will it only increment once? Some more information on how to handle this would be great.