The Lumia 620 is calling me and I have some questions

I'm currently using an HTC Radar running Windows Phone 7.8 and I really love the phone and OS, it's just that all the apps these days are coming out for Windows Phone 8. Looking for something roughly the same size I came upon the 620 as the ideal next Windows Phone to get. Things I like about it: compact size, cool color choices, expandable memory.

Questions:

What apps (specific ones if you can name them) will not run with just 512mb RAM?

Anyone know a reliable source to get the back covers? It's so easy to find the black 620 on Amazon, but I want a white, cyan or green back cover.

Took a look at GSM Arena and it has the following bands for HSDPA: 850 / 900 / 1900 / 2100. As I understand it the 1900 has been refarmed by T-Mobile in many areas, NYC where I live included. 1) This should work right? and 2) Does anyone have any experience using the refarmed 1900 band in NYC (is it reliable, etc)?

The iPhone and to an extent Xperia Z, are calling out to me as well, and the next iPhone should be out when my contract ends (technically I'm able to upgrade now but it's not worth it for me yet). Would you just wait it out and buy the iPhone?