why upgrade when there's no need? there aren't really any new features that are significant.
plus, PHP is not yet marked as "release" with Apache 2.x. PHP kinda sucks - ok more than kinda - but a lot of people want it to work, so upgrading now would probably not be wise if you need PHP.
i'd stick with 1.3.26. it's fast, it works great, it's compatible.
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