What graphics card should I get (for Sims 3)?

I'm really excited for Sims 3 Supernatural, the only problem being that my graphics card right now is less than stellar. Whenever I play Sims 3 (just the base game with no EPs) with anything else running in the background, my graphics card temporarily fails, and the screen goes completely black for a while until it begins to work again. I'm worried that, by installing a new EP on top of that, it will do that all the time, even when my game is the only thing playing. I'm also a bit concerned that it may make my graphics look terrible, which I also don't want. But mainly I'm worried about the graphics card crashing.

Rather than cutting Sims out of my life (which would kill me slowly) I'd like to get a new graphics card. I don't know much about computer hardware, so I need some help deciding which one should get. It doesn't have to be too amazing--all it has to do is run Sims 3 plus a few EPs without crashing and with okay and perhaps even good graphics--but I would really prefer one that isn't going to cost a ton of money. I'm sixteen, so I don't exactly have a lot of cash to spend, and I don't want to spend a ton of my parents' money, either.

Here's some information about my computer that I hope will help:

ASUS Notebook K52F/K62F

Intel Pentium CPU P6100 @ 2.00GHz Processor

3 GB (2.86 GB usable) Ram

64-bit Operating System

And this is a bit about my current graphics card:

Intel HD Graphics

1270 MB of memory

Thanks in advanced for any help.