+1 @jeffredo. We're not sure that's a decent 400W PSU (such as the ones Dell uses), it may be an utterly cheap generic, that will smoke if attached to a 4850. Without knowing more about it, the safest choices would be the low power nVidia 9800GT he linked, or an ATI HD4670.
Virajk, can you post what is on the label of your PSU? Brand and model would be nice, but at the least there should also be a table listing its outputs.