Pulling this thread back on topic before it becomes a distro x vs y thread :p the real answer is that the question is a bit broad.
So far as a distro goes, you want to choose a stable one. This means realistically you're looking at Debian Stable, CentOS, Ubuntu Server or Red Hat if your happy to pay for the obligatory support license.
If you like to live on the cutting edge and can live with things breaking now and again then look at options such as Fedora, Debian Testing or Sid, Arch or Gentoo
The above choice is a trade off between having say the latest version of php or not, and personal preference in terms of layout and tools (eg yum or apt-get)
Now you have your OS sorted you can worry about servers. Apache is probably the most popular webserver around, though if you are tight on resources you should have a look at lighttpd.
If you need a database server mysql is generally the standard in my experience however postgesql is another option.
Sendmail is the obvious choice for your mta and most scripting languages have decent modules to help manage it.
Hope that all helps