[DaiTengu]

I've always been partial to Gentoo as I'm able to squeeze a bit more performance out of it. If you're not a Linux guru though, it probably isn't the best solution.

Most dedicated servers offer Red-Hat based distros (RedHat, Fedora Core or CentOS). I'd go with one of those, or Debian if you're feeling a bit daring.

How many users your system can handle really depends upon a few factors. First, the more hacks and addons you have installed the higher the load will be on your server. Second, larger forums with many posts in the database will cause your SQL server usage to be higher due to searches. Third, if you do manage to use all your available RAM, the type of hard drives could cause a slowdown too. For the best performance you want SCSI drives in a RAID 1 or better array followed by SATA drives. Standard IDE drives will slow your site down if it starts swapping to disk.

I don't forsee you having a disk swapping issue, as your CPU will give in long before you're out of RAM