Maybe you should consider buying a desktop computer next time, because they are generally more price worthy, and also they're not as likely to get overheated. They have much larger screens too, and better keyboards, actually I think everything about them is better, except portability of course.
4 GB RAM isn't really enough for Sims 3, so if you can afford it, 8 or 16 GB would be much better. RAM isn't very expensive nowadays.
You also need a good graphics card with at least 1 GB of dedicated memory. If you don't know all that much about computers yourself, try to find someone who does, who could help you with your purchase.