For Unix-like platforms, I don't think it makes a difference which you upgrade first. To be clear, you don't really upgrade any of this, you just install the new software (10g in your case) into a new O_H and then deinstall the old versions once you've upgraded the database(s) using the old O_H. As for IDS, I've not used it, so I'm not sure if there are any special considerations related to it. Typically, I don't see folks installing IDS on database servers (even in development).
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