I'm not that big, only 300 members and only about 10-20% of those are active visitors. I max out about 10-15 people at any given time and have not had problems with Access BUT if I had to start all over again, I would have used SQL7 before doing all these mods. My current project is using SQL7 and it seems so much faster because it is a true client/server connection.
I am using Access on my personal web site and MS SQL 7.0 on the company's Intranet. My policy is that to test and experiment on Access and then build the real thing on MS SQL. It's more robust and designed to last. Remember that Access was designed as a desktop DB. My 2 Cents.