Well, Dev-Team just released both RedSn0w and PwnageTool to jailbreak ATV2 4.3.1. So you may want to give that a try instead of GreenPois0n because RedSnow and PwnageTools are untethered jailbreak. If you are set on sticking with ATV2 OS build 4.2.1, then you can use any of the jailbreaking tools: GreenPois0n, SeasonPass, RedSnow, PwnageTool.
Remember, if you do decide to have your ATV2 on 4.3.1, you'll also need to use the latest nightly build of XBMC in order to have accelerated playback. The current official XBMC release does not have the fix for VideoToolBox on ATV2 running >4.3.
Lastly, forget the browser. You want to catch up on missed TV episodes? There's a better way. Let me enlighten you...Look into the icefilms addon. There's a thread asked from a "noob" point of view with instructions on how to get it installed here:
http://forum.xbmc.org/showthread.php?tid=98396
To quickly answer the other questions to pose:
1. Know which iOS version you are running and understand the difference between your ATV Software version and iOS version.
instructions: Once you get your ATV2. Hook it up, go into settings/about and see what iOS version you are running. Press the center button on your remote while on the about screen and you can toggle between ATV Software version, Build and OS software. OS Software corresponds to iOS software. Here's a quick table:
ATV2 Software 4.0 = iOS 4.1
ATV2 Software 4.1 = iOS 4.2
ATV2 Software 4.1.1 = iOS 4.2.1
ATV2 Software 4.2 = iOS 4.3
ATV2 Software 4.2.1 = iOS 4.3.1**
**note: Apple TV likes to drop the last digit and simply display OS version as 4.3
2. Assuming you got a pre iOS 4.3 device, you can decide to stay on the lower version or upgrade to the latest 4.3.1. The untethered jailbreak for 4.3.1 just got released a couple of hours ago. The safer route is to stick with 4.2.1 for the time being.
Regarding your question about NitoTV. GreenPois0n will automatically install NitoTV as part of the jailbreaking process. NitoTV is adds a additional menu onto ATV2's Lowtide interface that allow you to install other scripts, addons and updates, all from the convenience of the graphical interface. It's definitely a nice feature to have, especially for newbies who aren't familiar with commandline stuff. From the NitoTV menu, you can easily install the latest official release of XBMC. Like I mention earlier, the current official version of XBMC works great on iOS 4.2.1 and below.
Anyways, this is getting longer than I intended so I'll stop here. Good luck.