Are Android updates hardware-limited?

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http://androidforums.com – Greetings, I'm new here so forgive me please if I'm in the wrong forum with this question, but I thought that it would probably be an newbie FAQ and I don't find it here. Are all Android phones able to accept the most recent Android release, or are they somehow version-limited due to hardware or other constraints? If I were to buy a GSM phone with Android 2.3, and then unlocked and rooted it, would I be able to download Android 4.2 from Google and placce it on the phone? I'm very new to all this, and feel that I need to get some basic info before I ask other questions about particular phon (General)