OS won't matter for performance. Don't worry about that. (maybe. Consider it negligible all other things (ram, hd, 32 bit vs 64bit) considered)Also, consider that the latest released version of an OS could be better able to handle system patches for a longer period of time than the "stable" version you might consider. Check the End of Life support for more information. What you don't want to do is to install something "stable" that "works" but you have to rip and replace the OS when you're ready/forced to upgrade because the patches for the OS aren't there any more.
From what I've encountered, though, I just upgraded through natty to the latest version of Ubuntu on my (crappy, home, non-production) system and aside from a small tweak in apache, I was back in action. (hours of upgrade of OS notwithstanding). Even upgrade MySQL 5.1 to 5.5 seemed to be okay.
Without attachments, your database will grow relatively slowly. With attachments, and if your db handles attachments in database instead of File System, your db could grow quite large. How large? depends, of course, on attachments and how many, but in your smaller (?) environment, you might reach 1gb in a year or 2.
The VM (virtual box) I had distributed for 3.0.10 was less than 2GB including OS. You should consider installing the server version of your OS of choice instead of the desktop version. You don't want or need, necessarily, X-windows/Gnome/wobbly windows, Open Office, games, etc.
How much hard drive space? Again, depends on attachments. For your environment, I would say you could start and be happy with 20gb for quite a while.
Another option I'd consider suggesting is installing
automysqlbackup which does replicate your mysql backup, but it enables you to have monthly, weekly, daily backups of mysql with little effort. You'll probably still be okay with 20gb, but 50gb may be more breathing room.
Hope this helps. More RAM is better, but for your usage, whatever you throw at it will probably be adequate. Back up early, often, and maybe throw Nagios on your system to monitor everything.
Your results may vary. Your specific needs may exceed examples expressed here. Don't rely on this example as typical or even legitimate. Whatever you think is needed, more is okay, and probably cheap enough.
