Help! What's my ideal set-up?

Let me start off by saying this, I am completely lost! It's been many years since I've taken a serious look at the hardware side of computing (finally time for a major upgrade) and I last was on the scene when SATA started to come about. Since then it appears quite a lot has happened (and is happening, flash comes to mind), and I was hoping someone could help me out a bit or point me to a website with (perhaps less sophisticated, more novice friendly) information regarding current HDs.

I'm starting to gather information to build a new machine (just looking to see what's available, mixing and matching things, I'll probably actually buy something late January to mid March). A few things that confuse me; what's the difference between a 3.5" and 2.5" HD, what exactly is AHCI (I think it's required/used in all SATA setups?), and is NCQ a big deal (something to make sure to get)? 7200 RPM or 10,000 RPM, SAS or SATA? Another big question I have is should I get two hard disks (one for OS and related 'essential' use programs) and another one to dump games in?

I'm building something more towards games (generally that's what I use my computer for, in addition to the typical web-surfing, mp3 use & ripping, and work related use), ideally I'd like total storage around 250-300 GBs (I generally like to leave a few games installed at once as my tastes vary and I find myself skipping around game to game). I have no experience using any sort of raid environment and would prefer to avoid it if possible (unless there are substantial gains to be had).

If anyone is able to help, offer advice, I would really appreciate it. I know I asked some pretty broad and heavy questions so if anyone is able to offer a newbie-friendly website or resource I would greatly appreciate it!

Thanks for the input g-paw, I appreciate it!