It's true that 512mb vRAM is the minimum for most of today's games, but using 1680 x 1050 and higher resolutions with AA/AF/etc on, you'll need 1GB of vRAM. 2GB is wayyy overkill unless you do game at 2500x1600 resolution and even then, there isn't a big difference with the 1GB.
Here is an article from Tom's comparing the 512mb, 1GB, and 2GB versions of the ATI 4870, just click here
Edit: ATI's #1 benefit is that you get the best performance for what you are paying. With equal performance to Nvidia cards, ATI cards will cost just slightly less, as you get into the higher-end cards, you'll see a big gap in price when you compare an ATI and Nvidia card.
Not a fact, but people say: ATI GPU seems to do best with an Intel CPU and a Nvidia GPU seems to do best with an AMD CPU.