I'm looking for a program that allows me to offer webmail to my users.

Does Neomail offer that??

How much extra load does this put on the server?

Neomail is one of the easiest to install webmail program you will find.

I was somehow worried about installing it on my raq3, but it really works like a charm, and has multiple language interfaces.

The only "against" point for some users is that it does not support html-mail (and it will not support it).

For me it's not an issue, the interface is clean and clear, and I noticed no significant extra load. Anyway, not every user in the server will be using webmail.

Try for example:

http://jawmail.sourceforge.net/
http://www.horde.org

Or do a search for squiremail