Forgive me if I am obtuse about this, I am a technological newbie -

I recently purchased an unlocked 7100g on e-bay. Will be using on t-mobile. Phone does work with SIM card. How do I determine what operating software it has & can I upgrade using the t-mobile upgrade? What the heck is enterprise? Does the 4.0.2.49 upgrade on t-mobile have a more "user-friendly" look than what we are currently seeing? My husband has a very old Palm Treo that he LOVES and it would be great if this screen looked more similar.

Wirelessly posted (8700g)

Select the options (or settings) icon. It may be in a folder. Then select about. You will see your OS version on this screen.

Did you really mean 7100g (and not 7100t). I'll defer to others. I'm not really good with different models. Is the 'g' a GSM device? If it isn't, then you will not be able to use it on tmobile. EDIT: I see the 7100g was the AT&T version of the 7100.
What you call enterprise is BES. See BES, BIS etc at blackberryfaq.com.