If you elect to proceed to block/pass by IP address ranges, I would recommend using PF rather than Apache, as your concern is resource consumption and blocking addresses at the kernel would be more efficient. Apache would not receive any blocked requests.
Please be aware, OpenBSD 5.0 is no longer supported. Support ended 1 Nov 2012, when OpenBSD 5.2 was released. Only the most recent two releases are supported. The most recent release was OpenBSD 5.3, released 1 May 2013.
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