Glad to see someone using OpenACS for underfinanced NGO work, this is exactly the target space I had in mind when I got involved in the project, having spent about twelve years on the board of one of the largest conservation organizations in Oregon.
AOLserver 2.3.3 supports virtual hosting, which lets you host multiple virtual servers on one IP. Direct support for this was removed from AOLserver 3.0, but there are a couple of user-generated solutions for this. The best places to look for more information are in the "virtual hosting" AOLserver forum (http://aolserver.com), and at web/db, now at http://arsdigita.com/asj.
