Why do you want to add a graphics card? If it's simply to get more video output options, then you might be on the right track. If it's to improve gaming performance, you might be disappointed with the minimal gain any card of this type will give.
If you're not computer savvy and you want something that's just going to work, you might want to reconsider this. If you're willing to experiment a bit and have the time to troubleshoot any problems that might occur, go for the 5450.