Market shares tell only a part of a company's overall net worth. Share volume, outstanding shares, market value, etc all play into the bigger picture. I worked for the American Stock Exchange for 3 years on 86 Trinity Place in lower Manhattan as a market data analyst and know a *little about this.
Nvidia's share volume is 25,878,955 with 165 million shares outstanding. Price is 10.00 a share. 10x25,858,955= $258,589,550.00
On the other hand, here is a little of Ati's info.
The share volume of Ati is a mere 5,699,697 with 245 million outstanding. Price is 14.68 a share. 5,699,697x14.68=$83,671,551.96
Outstanding are unpurchased.
In perspective, if Nvidia had the same share volume (which by the way, is FIVE times larger) as Ati, each stock would be worth upwards of 50 dollars each as opposed to 14.68, going strictly off the current index.
You cannot look at price figures or market drops and percentages as clear indications of a companies net worth, profit margin, or position. The market does not function that way. Nvidia clearly holds a larger portion of the market, regardless of individual share price or percentage flucuations. Due to it's volume, it would have a greater influence on the NASDAQ's daily earnings or losses than Ati.
In the end, it wouldn't make any of the current NV40 or R420 cards perform any slower or faster for that matter.