I suspected you might already be familiar with them, given the traffic levels of your sites, but I thought they were worth mentioning.
There aren't any hard resource limits as such, other than the overall space and bandwidth limits of your plan, but DreamHost will act if your sites are having an adverse affect on other sites on your shared server.
This doesn't mean that your sites will automatically be taken down if this happens, generally you will be notified and asked to reduce your resource usage. Your sites may also be moved to a temporary sever until your resource usage is reduced.
The actual level of traffic you can reach before others are affected is obviously going to depend on many factors, such as the nature of the pages served, the resource usage of others on your server and a myriad of other factors.
One thing worth mentioning; DreamHost does offer a 97 day money back guarantee (minus the $9.95 registration for your 'free' domain) if you pay by credit-card, so you could always try a few sites here and see how it goes.
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