Conrad, how large are the threads sites? A celeron would be fine for a small, threads site, but these are really not server chips at all and will not handle higher traffic, let alone multiple sites.
When my site was a little smaller (1200 members, and about 700 posts per day) it was running on a dedicated P3 1.7ghz machine. It was adaquate, but when a lot of users were online (more than 50) you could really start to see the server struggling.
So it all depends on the size of the sites, but in all honesty I would at least try to avoid celerons for servers, especially for a dynamic platform like threads. You will leave yourself with almost no room to grow.