Q: Does Plesk install PHP4, or PHP5?
A: Plesk doesn't really distribute, or create core packages like httpd, php, mysql, bind, etc. It is designed to use whatever the vendor of that OS distributes. So in the case of CentOS 4, that would be PHP 4.3. Plesk does mirror core OS components from the vendor, which will install the vendors packages required to support Plesk.
 Q: Why doesn't Plesk distribute these packages?
A: In 2000, while at Plesk we (Two of the Plesk founders founded Atomicorp, Scott and Mike Shinn) made the design decision to leverage linux vendors packages where we could, rather than distribute our own. This was indended to both open the product to 3rd party enhancements, and reduce the training overhead required to manage plesk servers in a hosting environment.
 Q: What versions of PSA are compatible with Mysql 5.0?
A: PSA 8.1 and above.
 Q: Can I get PHP 5.2 and MySQL 5 on Plesk if I'm using CentOS 4, or RHEL 4?
A: Yes, however it cannot be installed through the plesk autoinstaller. CentOS 4 users can install PHP 5.1 and Mysql 5 from the centosplus channel. RHEL4 users can also use the CentOS channel, by adding it to their up2date configuration. PHP 5.2, and Mysql 5.0 are available in the [atomic] channel.
 Q: What Operating System do you recommend, and Why?
A: CentOS 4 or CentOS 5. They have 7 year support cycles, and the largest community focusing on server packages. If you require paid support, then we recommend you use RHEL 4 or 5. For a business environment, Centos or RHEL are ideal.
 Q: What should I look for in a hosting company?
A: Stable, Consistent bandwidth.
Good hardware SLAs, the manufacturer doesn't especially matter. You just want to know your hardware will be quickly replaced when it goes bad.
Serial console access, *not* an IP KVM. Serial consoles have a lot less that will go wrong, and you can access them from anything (including PDAs, or even a Sidekick II phone). IP KVM's are generally java based, which limits your access options. You can also automate things over serial consoles (like forensic tools, or logging) that you can't over an IP KVM.
PXE based provisioning and rescue.
2 or more disks in the server, so you can create soft RAID mirrors. This your most likely hardware failure.
 Q: Every time I run yum upgrade, it changes my MTA from postfix to qmail, or qmail to postfix
This is due to a plesk bug in the psa-mail-driver-pc and psa-mail-driver-qc packages. By default if you set up the plesk repo using the atomic installer it will add an exclude=psa-mail-driver-qc to the plesk.repo file. You will get this condition if you created your plesk.repo outside of the atomic installer, removed the exclude line, or are otherwise overriding excludes with --disableexcludes=all.