I strongly agree with doing as pornguy suggested, and as google, yahoo and other huge sites do.
You'll notice Yahoo separates their site into logical sections such as mail.yahoo.com,
groups.yahoo.com, news.yahoo.com, etc.
There are sections called adwords.google.com, news.google.com etc. There is no such
thing as graphics.google.com, scripts.google.com, or html.google.com. There are several
very good reasons for that. A site divided into logical sections like tour.site.com and
members.site.com is more reliable, maintainable, and secure than one splintered based
on file type, with html.site.com, gifs.site.com, and etc.
In short, to follow the lead of great companies like Google and Yahoo, make
www.site.com
and members.site.com, NOT html.site.com, phpscript.site.com, and jpg.site.com.
Another reasonable split is implied by the names you see but never thing about.
You've seen server names like mail.site.com,
www.site.com, ns1.site.com, and
www.site.com. That's reasonable to have different servers perform different services.
Having a database server is very common, seperate from the web server. That database
server could also be a mail server or a DNS server.