Running software for website

So say i wanted to turn my site in to one of those video upload sites like youtube or something. or basically anything that would require running a program(to convert videos to flv in this case). what would my webhost need to support in order to do this?

>>5
PHP 4.3 is severely outdated, strongly recommend you use PHP 5.2 (lang/php5). Same deal with MySQL -- consider installing databases/mysql51-server instead of using an old branch.

Really though, if you're asking for such general help, I doubt any of the technical details are going to matter to you -- you're probably better off looking for a canned software package which does what you want, and then installing what it needs rather than guessing ahead of time.