Okay, first of all, you should upgrade to at least 1.4.0sr6. It shouldn't break anything(*) but will give you an extra bit of security. But then again we're talking about a version that's over 3 years old and not supported any more. Some of the recent security issues may also lurk in that version.
As for your actual plan: Geeklog 1.4.0 should work with PHP 5, without any special requirements (no register_globals or register_long_arrays needed). It worked with the PHP versions current at the time. Wether or not it works with PHP 5.3.0 - no idea, frankly. I can't think of a reason why it shouldn't but can't guarantee it.
bye, Dirk
(*) Strictly speaking, it will break uploading through the old version of FCKeditor included in that version. But that's a security fix and therefore a good thing ...