2000 dollars on a video card? you are hard pressed to spend over £300 on a single card really. For that kind of money, you are probably talking about a whole set of possible upgrades, not just the card. Hell, you could squeeze an SLI upgrade perhaps (two graphics cards in one system for more grunt)
Now, the nvidia/ATI thing is practically a theological argument for a lot of people. People have their favorites, and honestly, half the time they dont have any real reason for choosing the favorite.
Right now however, Nvidia probably have the edge, simply because of the avaliability of SLI systems. This doesnt mean you shouldnt consider ATI though, but you need to judge each card on its OWN merits, not judge it by the brand.
There ARE no third parties that are worth a damn, and its been that way for a long time. You see, your ATI or Nvidia card just uses the GPU from the respective manufacturer, and the rest of the card is made by somebody else. So say an "MSI Geforce 6600GT" would be an Nvidia chip, on a board designed and built by MSI, and may or may not have the same specification and performance as another card with the same GPU, by a different company.
You get companies like gainward, who slap some faster memory on the card to make it perform better, and crank up the price accordingly. Between manufacturers you can have differences in GPU speed, Memory type, quantity, and speed, and differences in the cooling systems used.
Also, ATI and Nvidia use contractitory naming schemes. If an ATI card has "XT" tacked on the end of the model name, thats the superdooper version. If an Nvidia card has that suffix, that means its the crappy cut down version.
I think the first thing to decide, is are you spending the right amount of money, and in the right place? I dont know the exchange rates, but it sounds like a lot of dosh. Maybe your going to need other upgrades to get the most out of a new graphics card.