The ATI x1600 is a good entry level graphics card. It has the ATI label, which means it will be a stunningly colourful experience. The difference between 512mb and 256mb is the size and amount of textures you can hold in memory on the card. If it's a new game, it will have higher quality textures reserved for 512mb cards and it will look more detailed. But you need to remember that this is an entry level card and might not be able to perform as well if it has larger textures.
The nVidia 6800 is a good card as well. It was the high end nVidia card for a while, until the 7800 came out. But you have to keep in mind, that the x1600 came out around the same time as the 7800 (a bit after), so it may very well be based on newer technology. (I haven't read any reviews on this).
Out of these 2 cards, I would choose the ATI, purely because of the quality of image you would recieve.
Have a read of this website, it will give you more then enough information about the x1600
http://www.tomshardware.com/2006/06/...eon_x1600_pro/