I'd also recommend looking at a prebuilt PC over a build your own, it will likely be less expensive and the components you need will all be included.
I don't know what other games you play but The Sims isn't a game that needs a bleeding-edge game rig so that will keep your costs down.
You need a processer that doesn't crawl (This is one I've seen lately that is problematic, they have some really pokey processors out on the market now that my older rig blows out of the water), enough RAM for programs to run nicely, enough Hard Drive space to hold a lot of data, and a decent graphics card. You will also need a monitor that has a good picture quality and is of a size that works best for your visual comfort.
