What kind of video card should I buy?

I just bought the sims 3 and my computer wont let me play it. A screen pops up saying that i need to get a new video card, I have a dell Dimension DIM 4550 so what kind of video card should I buy. I kind of wanted to keep the price down because all i want to do is play the sims with it. So where can i buy them and what kind should i buy? details please! :)......|||Your system comes with a 250 watt power supply making your choices very limited for a new video card, you may have to upgrade your power supply then purchase a new video card.