I was on deployment when I planned building my first PC. That gave me plenty of time to compare parts and delay purchasing until I came home. www.pcpartpicker.com is a great resource for putting a build together because it will list various prices, discounts, and offers from numerous online retailers. Also, you'll more than likely have a list of compatible parts, but it's always a good idea to double check. I think 8-12 GB of RAM is a good choice. I've been at 8 GB for 2 years with no noticeable problems or serious malfunctions. I don't know if this is possible with mid- or regular-sized cases, but I thought it would have been oversight if I didn't make sure the mobo would fit inside and seat properly inside the case. I picked up a power supply with an extra 200(300?) volts for upgrading.
It's almost been two years, and my PC has been running fine playing most games on ultra without problems. I'm not a major programmer or video/audio editor, but I've shared my computer with friends with them having no problems at all. In a couple of years(or if I had the money now), I think some things I would do differently would be using SSD instead of HDD, getting a motherboard with the appropriate PCI slots so I could dual SLI, and just buying a wireless network card (for a while I bridged the network through ethernet with my laptop which was really difficult and took me months to discover there was an easier way to go about it).
Unthinking respect for authority is the greatest enemy of truth. -Albert Einstein