PlainSite (http://www.plainsite.org) has about 12K users per day on average. Most of them are just browsing for cases, not paying, but some are paying.
The main stress on the system is really determined by the complexity of the SQL queries on each page. I've spent a great deal of time optimizing them, and I know there are certain ones that need to be further optimized. I have the database (MySQL) on one server, the web server and documents on another, and static resources such as images on a third, which probably isn't even necessary. All three servers run Linux and the database server has 48GB of RAM. They're hardly new; you could buy all of this equipment today for under $1,000 total.
The biggest technical bottleneck is really RAM; the biggest expense for this kind of site is bandwidth.
The main stress on the system is really determined by the complexity of the SQL queries on each page. I've spent a great deal of time optimizing them, and I know there are certain ones that need to be further optimized. I have the database (MySQL) on one server, the web server and documents on another, and static resources such as images on a third, which probably isn't even necessary. All three servers run Linux and the database server has 48GB of RAM. They're hardly new; you could buy all of this equipment today for under $1,000 total.
The biggest technical bottleneck is really RAM; the biggest expense for this kind of site is bandwidth.