Congrats on the sale and thanks for all the details you gave. You should send the sale info to DNJournal for their weekly sales report.
Social Gaming is really getting big on the net, and gaming related domains are selling for good amounts. Part of the reason is that many of these games sell virtual goods for real money, and don't need advertisers. This domain could be for something similar to 'Cityville'.
Besides being fairly firm in your negotiation, what also struck me was how quick it went (3 days?). I've had negotiations where the price separation was only a few hundred dollars and it drags on for weeks!
I think it also helped you had a buyer who wanted that exact domain and had deep pockets.
MegaCity.com is a nice brandable domain, short, easy to spell and remember, I can easily see it being used for a gaming or social type site. "Mega" is a popular word in the UK, kind of like how "awesome" or "super" is used here, or at least it used to be.
How did you used to handle these types of enquiries, before becoming a "hard ass" - lol