Buy a ATI Radeon HD 6770 if you don't want to spend much money,otherwise a Radeon HD 6850 and you will be good for the newest games and the games to come in a couple of years at high specs.
If you want a nVidia card go with a GTS 250 or spend some more money with the GTX 460.Both of them are on the same category with the ATI ones respectively.
Also,all the cards need a PCI Express x16 port, so be sure that your motherboard(which you should have mentioned her model) has one.I am saying this because the 4200 is an intergrated one so when you will be doing the swap don't expect to find a card.You don't need a lot of memory 1GB is super enough for only one monitor.Above is efficient when you have three or more monitors.
To end with, you can visit
http://www.game-debate.com/ search any game and if you scroll down you will find an area to submit your hardware and find out if you can play the game.This way you may find a cheaper graphics card that will do the job you want.The ones I suggested are capable of running heavy games like Call of Duty Black Ops, Battlefield Bad Company 2, FEAR 3, Dirt 3.
