Hi, Janna.
The processor seems to be pretty good. We have an Athlon II x4 630 from about the same time period and the Phenom chips are generally better overall than the Athlons. So, no problems there.
I took a look at a
review for your ATi Radeon HD 5450 card. It's available with up to 2 GB of dedicated graphics memory. Unfortunately, it's a bit like the nVidia GT line of desktop graphics cards in that it's a budget card designed for online surfing, video streaming and other light tasks, and not for gaming. For improving your performance on your current setup, you'll need to try lowering the resolution and graphics details - turning off the Advanced Rendering in the Graphics Options can really boost performance.
If you're wanting to run everything at the highest settings, I would suggest a stronger graphics card. Along with that you'll likely need a new power supply as Dell, HP, and such are notorious for using the bare minimum power supply necessary.
For a graphics card, I'd recommend one of the nVidia GTX series cards, but not the latest and greatest (as that will set you back a
lot of money). Instead, I'd focus on the GTX 600 series as they are much cheaper and still excellent cards. The card MrsFlynn and I use is the
EVGA GTX 650 Ti and it works very well. The
1 GB model should also work fine.
Granted, these are a bit overkill for Sims 3, but they should handle anything you can throw at it graphics-wise for some time to come.
For a power supply, I would look at about a 500 - 600 watt supply. Again, overkill, but you want to make sure you have your bases covered. Actual brand and such doesn't matter too much, just get something you can afford and has a decent warranty.
Ask your friendly neighborhood computer geek (sounds like your son would be a great candidate) to install the components for you and to help you shop around for prices.
Hope this helps, and happy Simming!
Flynn