nVidia doesn't make the cards. Of the card manufacturers, eVGA offers
direct-purchase through its web site. Not all the cards are in stock,
though, especially at the high-end.
ATi typically prices the cards 13-20% higher than other reputable online
retailers, and sometimes the flagship cards are not available due to limited
supply. From what I've seen ATi doesn't package the item any better or offer
better after-sales support than leading merchants like Newegg or NCIX.
In 2003, I bought a Radeon 9800 Pro 128 MB from ATi directly as part of a
trade-up program. Before ordering, I called ATi to ask a few questions, and
was quickly connected to a English-speaking representative. The card shipped
quickly enough and I was sent several emails along the way informing me of
the progress of the order. This is no different than any other reputable
merchant.