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I couldn't imaging owning any android or Linux for that matter and not have root, So I guess I don't understand your question, can be upgraded means the hardware is sufficient to support the latest OS, and there is a rom be it official or unofficial that can be installed on the tablet. , if the tablet can't be upgrade then no I wouldn't buy it,For the cheaper tablets, I get the feeling that the manufacturers don't have an official upgrade plan; they simply release a new model of the tablet with a newer version of Android on it. Would upgrading such tablets involve rooting it and flashing a 3rd-party image/ROM ?
From what I hear, Android 2.x is suited for phones. Android 3.x is for tablets only. Android 4.x is for both tablets and phones.
I find ICS 4.0.3 on my tablet much better than GB 2.3.4 on my DroidX, but Motorola/Verizon may be causing problems on the latter.
Android changelog: Android version history - Wikipedia, the free encyclopedia
According to this Android developers statistics, Android 2.3 OS is installed in 64.6% android devices.