Difference between ati graphics and nvidia graphics

They are 2 different companies.
They make different cards.
Some games play better on one type, some on another.

In general terms ATI is currently ahead at the high end, but Nvidia are releasing new cards within 2 weeks. Both companies have good products at different price-points depending on your needs.

What PC do you have?
What games do you play?
What is your budget?
What resolution is your monitor?
 
Ati is a bit cheaper than nVidia although they have usually a worse performance (but nowadays it´s not the case)
In my opinion, nowadays, the Ati Radeon HD5770 is the best option (it isn´t too expensive (150€) and has a good performance). But if you can afford the price (240€), the Ati 5850 is great.
 
I just put a xfx 4890 in my newest computer. it runs crysis fine. it had issues at the end of the game but i just had to reinstall the newest drivers. is the 4890 a great card?
 

what is Dx11 and is it that big of a deal
 
atchuta - Please be more specific. What sort of graphics are you asking about? Are you asking about playing games or are you asking about professional applications? Without additional information it is not possible to make a valid opinion.
 
I agree with Johnny, mostly... I say "mostly" because both companies offer similarly performing cards in similar price brackets, and often trade blows until you get into the upper tier(s). Though, I would say ATI holds a slight price/performance edge in most cases.

But, yes, a recommendation does depend on his specific use/application.