|"Do I need to buy a ATI graphics or NVIDIA for playing latest games?"|
Yes, you need to buy a video card. Either ATI or nVidia will work, but you have to check your power supply to see if it's capable of handling a gaming card. And if you have a small form factor case, you will have to get a low profile card. We need to know the model number of your PC to be able to help.
Best Graphics Cards For The Money: November 2010