I would have to agree with PCS-Chris, but wanted to ad something. 100 sites is fine as long as they are static sites, but any amount of traffic coming from one or more of the sites may slow down the server. If you expect anything to happen with any of those sites, an upgrade to a little better Dual Core cpu would not be a bad idea.
This all depends on the combined resources used by all of your sites. Like jeremy said, 100 static sites with normal bandwidth would be fine.
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If you are hosting only static HTML type sites, you might also consider using lighttpd or LiteSpeed on your server - an Atom 330 CPU can be stretched a LONG way with a lightweight httpd server like those.
Lighttpd or LiteSpeed work great for dynamic sites too, but too much DB activity will bog down an Atom, even running one of those httpd daemons.
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