So I rebuilt the computer today, and it came out great. Maybe some of my hardware choices weren't the best (overkill) but at least no one can say its a bad computer. I ended up going with...
CPU: i5-4430
Mobo: Biostar
RAM: 8GB
PSU: 600w Corsair
HDD: reused 500GB SATA
case: reused old case
OS: Win7 Home Premium 64bit (upgraded from Vista 32bit, yuck)
The CPU might seem too expensive to justify in a computer for Mom, but it wasn't too much more than the i3 that I would have gotten, and NewEgg also gave me a promo code to save 10% on it, making it even closer in price. I'm sure it'll at least come in handy when encoding video.
The Corsair 500 and 600w PSUs were the same price at the time of purchase, and not that much more than the 430w version I was gonna get. On top, there was a $20 mail-in rebate for a Visa pre-paid card. Actually, now that I'm thinking about it, I forget if I got the 500 or 600w version lol. But I decided to go with a little more wattage since they were essentially the same price. I just looked at the prices now and they jumped up. Either way, more than enough power for now. But that's good in case I end up adding anything like a video card down the road or additional hard drives or any add-on cards if they become necessary. Hell, maybe I'll even max out the CPU something like 7 years from now when they're dirt cheap on eBay.
The 8GB might have seemed like overkill but it's between 4 and 8 and it's hard to go with just 4 these days. Especially since she's been using Windows Movie Maker, which can load a lot of shit into RAM. Also, like I suspected, I had the option in the BIOS to dedicate different amounts of RAM for graphics. I set it at the max, 1GB, since I have it. If I only put 4GB in, it would have been less which is no good. And I wouldn't have wanted to set too much aside for the graphics.
So it might be overkill, I know. If I went with the i3 and only 4GB of RAM I could have reused the old PSU and it all probably would have been enough. But for only ~$100 more I got what I got and I'm happy with it (even though it's not even my computer lol).