This is always guess work, but I would think you are in the ball park for the machine specification as long as it is a good quality server machine (other stuff in the spec makes a big difference too). I would however consider doubling the memory to 4G. You don't say what O/S you are planning on running, but let's hope it's Linux
We run 6 disks on our machines the first two are mirrored for the O/S and the remaining 4 in RAID5 configuration for the various data (and mysql
You will need to be fairly sure of your network connectivity. It really all depends on what sort of material the system will be delivering. At one extreme if it's all HTML then you don't have to worry. At the other if you want to serve multimedia then you do. What do you mean by 750 hosts? As you are talking about bandwidth I assume you mean that the Moodle service if going 'off-site' which makes your audience rather hard to pin down so specifically.