One of the most asked question related to OpenX ad server is: "How much load will OpenX place on my server?"
At the first place OpenX shouldn't be installed on shared hosting.
It's for sure that you will need to make some experiments in order to realize what configuration or hosting provider are the best solution for you. Luckily, OpenX ad server is free to use and it doesn't need to be licenced so you are free to use it on many machines at the same time.
It depends on type, size and number of ads the server will deliver and it is not easy to provide detail information about requirements. For examples, web sites showing up to 50 million page impressions for a month without any issue could be used for hosting OpenX installation. But if a server delivera billions of banners and ads monthly definitelly will need several dedicated servers configured for distributed statistics.