To get a good graphic card for that rig your PSU from 250w will come short of a nice GPU plus like said above how much can you afford? It really doesn't matter what your resolution is.
Unless you are going for broke then that rig is not the rig you will want. So bottom line your PSU will have to be upgraded and we need to know how much you can afford. To install the card here are a few links to do that. Good luck and get back to us.