Uptik,
I've been a computer geek for a long time and my record for building a computer is 14 minutes. (Is using an electric screwdriver cheating?)
Keep what you've got. The only thing worth upgrading for your uses would be adding memory.
The systems I use at home and work are much slower, and I can assure you that I use much more computing power than most people. I haven't upgraded them because there's no need for me to do so. No need for you to spend money on power you'll never use.
Learning to build them will take you about 2-3 hours for the 1st one because you'll be spending time reading directions. After that your speed will increase rapidly. Make sure you have bandaids on hand as it's common to nick a finger on one of the sharp welds of a circuit board. Also, although you can usually get away with it, it's recommended that you take the advice from the instruction book and unplug the cord whenever you have your hands inside a box. I once threw a sound card fifteen feet across the room when I accidently touched a capacitor.
And don't be shy about it. Components aren't as fragile as they look and it takes a little force to snap cards into the motherboard.