Which is better RADEON ATI or NVIDEA?

This is purely based on your personal preferences. Both the companies have pros and cons. Also if you want to use two cards(though not now you might want to upgrade later) you must check whether your motherboard supports nvidia sli or ati crossfire. Also you should take into consideration whether you want physx available only in nvidia. It would be more helpful if you could provide your system specs.

Actually, what do you want to do with your PC? If you just want to use your PC to play Games, Better for you to choose ATI. If you have unlimited budget to spend on GPU Unit, I suggest you to purchase ATI RADEON HD 5870. It will cost you for about USD 320 - USD 450. Or, if you have budget for about USD 150 - 300, it is wise for you to choose ATI RADEON HD 4890. See Passmark dot com for seeing the GPU rank. Or, if you have budget below USD 100, you can choose Radeon X1300PRO (mostly still use AGP Slot). But, If your PC activity is for design graphic, video rendering, and kinds of these application, better for you to choose NVIDIA GeForce Quadro. NVIDA Quadro is made to optimize graphic processing software such as Adobe Photoshop, Vegas Video Software, or any graphic rendering software.

I'm not much familiar with video cards. Both ATI and NVIDEA are the leaders in their fields. I think NVIDEA is better than ATI.

Well, NVIDEA is expensive than ATI, but if you ask my advice, i would suggest is NVIDEA if you have extra bucks. Have a nice day!!