WordPress sites can handle large volume of traffic when WP-Cache or WP-SuperCache plug-in is used. The second plug-in, SuperCache, is actually the WP-Cache+ some improvements to help sites deal with caching. When these plug-ins are used, cache pages are usually served to the visitors instead of extracting contents from database for every single visitor.
Well, that was not the case when I last used WP-SuperCache in the Dreamhost VPS. I don’t know much inside details but Dreamhosters need to find an alternative when WordPress Cache is concerned. Mine didn’t crash but there are reports of such things happening like this one.
For my sites I found Hyper Cache does some justification in reducing the server load. There should be others but I haven’t tried them. Please comment your experience if you have experienced such problems.