I, personally, am an Nvidia supporter. I have always liked thier cards and never had any problems with their products. My 460 GTX just died a few days ago and I had that card for over two years with a 20% overclock.
What is your motherboard specs? Is it SLI or Crossfire ready? If so, I would go with an SLI/Crossfire ready card. Budget seems to be a concern so some of the high end cards might be out of the picture but two cards in SLI/Crossfire could give you the performance boost you need.
This is the card that I am replacing my two 460gtxs with. Can't beat the deal. SLI Ready with about a 10% performance increase over 460s when in SLI.