Before I even took off the screen plastic wrap, I upgraded the memory with 32gb of OWC.
Congrats on the new iMac! Mine is out for delivery today too, very exciting day. From what I've read from other memory upgrade threads it seems prudent to ensure everything is 100% with the stock iMac (included ram) before then shutting down to upgrade it. Maybe you did do all this before removing the plastic
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In my case I have four sticks coming (also today) but I am likely going to be a little neurotic about it and add each matched pair one at a time in the recommended xoxo pair configuration. (I'll look it up when the time comes.)
The sticks are all Crucial and the same size and order, so there is very small chance of match issues, but something looks off at some point then doing it step-by-step it will have seemed wise. If not, I'll probably feel I wasted 10 minutes.
The mouse will take a bit of getting used to, and the keyboard is a bit different, but just a slight learning curve.
Are you coming from a MacBook and are accustomed to a trackpad? If so, I'd recommend the external Magic Trackpad if you can budget it – especially if you find you don't warm up to the mouse. I actually picked the trackpad in my BTO configuration as I didn't want the mouse – I have an old dusty Magic Mouse anyway. I've been using an external trackpad for years and (personally) prefer it. Bonus: unlike the mouse, you can still use the trackpad while it is charging.
(Yes, now I will have two trackpads but my iMac vs work MacBook need to serve different purposes and will both be regularly setup. I sometimes also use the trackpad from my iPad. Better two as I can keep them paired to different devices, rather than a mouse I'd never use.)
Anyway, enjoy your new Mac!