I'll try and answer a few of these for you...
I've been looking at desktops to replace my laptop....
I'm not very tech savvy, but I've read you can get better bang for your buck if you go barebones and put together the computer yourself. Has anyone here, who isn't all that tech savvy, done that?
This one I can't answer because I've been building PC's for 20 years.
These days, it's pretty easy though. Pretty much everything only fits one way. As a rule, if you REALLY have to force it, you're plugging it in wrong.
Also, which is better processor-wise: AMD or Intel? For AMD I'm thinking the 8320/8350 series and for Intel, the i5-4k series. AMD tends to be a little less expensive, but I'm kind of partial to Intel.
Intel is better for gaming. Don't bother with a K series processor. They're for overclockers. (the K means it's multiplier unlocked to make this easier, certainly not something I'd recommend for someone who isn't used to mucking around with a PC BIOS!)
For GPU, I've settled on the Nvidia GTX 650 1gb [which seems to give a reasonable amount of oomph] and 8 GB of RAM.
8GB RAM is good. I would also recommend the GeForce GTX750Ti. It's getting exceptionally good reviews due to the games it can run (Titanfall apparently runs close to maxed out in fullHD - 1080p), so it should eat Sims 3 for breakfast... and possibly desert.
Also, when's a good time to buy a new desktop?
I don't think there's really a "good" time to buy a new desktop. Prices are in near constant flux, so I'd say... set yourself a price limit, save up, and then get what you can at that time. Right now, RAM is fairly cheap and so are hard drives. But all it takes is a flood or a fire in a factory, and prices skyrocket.