What is the size of your power supply?
Specs say either 250w or 305w both of which are very small for running even a basic graphics card. The recommended size for the 4650 is 400w, although you could probably squeeze by with a little less but even the 305w might not be large enough, definitely not the 250w. The 4650 also requires a 6 pin power connector. Do you have one of those coming off the PSU?
For general guidance on selecting a video card, you might want to review upgradevideocards.com.http://www.upgradevideocards.com/index.html
What card were you running before?