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In addition to the hardware and software requirements, you will also need to think about the capacity of your Moodle installation in terms of the number of users it can handle. There are two numbers to plan for:
Once you know these figures for your users, you can start to work out if your Moodle installation can support this capacity. The exact number of users depends on your hardware/software/network combination. Usually the amount of memory installed (RAM) is the deciding factor but a faster overall processor speed will also help in reducing waiting times for pages to load.
The general rule of thumb for a single server is that the approx max concurrent users = RAM (GB) * 50 and the approx max browsing users = Approx max concurrent users * 5. As an example, a university with 500 total computers on campus and 100 concurrent users at any time will need approx 2GB of RAM on the one server to support the number of concurrent users.
[Update|Updating from a previous version] of Moodle has been improved quite a bit from 1.8.3+ version (see this thread discussion: http://moodle.org/mod/forum/discuss.php?d=83281). Moodle 1.9 has even more improvements in all areas and is the recommended version.