We're also running some installations our customer's own virtualised environments.

In one I'm working on at present, we're running moodle 2.1 with about 700 daily active users (sorry - no idea what the 'concurrency' is off-hand) on:
* 64 bit dual core VMWare vsphere machine
* 8GB RAM
* Debian Squeeze amd64 installation
* apache2
* apc

The box has a load average of < 0.3 at all times, and our apache2 threads seems to have a RES of between 15m and 77m at present. We have 400+ MB of RAM completely unused, and 7.3Gb in the cache. Only 680Gb is actively used.

Our databases are stored on a separate db server which is (unfortunately) virtualised.

Virtualisation shouldn't cause any major issues - though virtualising a database server isn't great.
Also, using a 64-bit installation won't use a significant amount more memory - it will likely have a slightly higher memory overhead but I believe that this is passed to the host, not the instance. Without a 64-bit system, and 64-bit kernel you won't be able to access beyond the first 3.6GB RAM (okay... you could be running big-mem, but the overheads are just as high, if not higher).

Andrew