6
To give you a example there are sites, much larger the XOOPS site it self, and XOOPS carries 13207 members as I post this.
One of my sites has over 3000 active members and the specs are:
# Intel Celeron 2.0 GHz or higher
# 256 MB DDR RAM
# 40 GB hard drive
# 10 GB traffic/month
I find XOOPS is very efficient and never causes my server any strain, except one module which is Flash Chat (Tufat) but this is nothing to do with XOOPS it's self.
As for Christian sites I seen quiet a few although I can't name them all, I think one is:
http://www.regenerateweb.nethttps://xoops.org/modules/news/article.php?storyid=166 (Don't think this one working now)
I've seen more but would require me to do some in depth searching.
Hope this is of help.