While ATI may have some cool features and stuff, I've seen some real issues with their cards too.
For example, they may be doing a good job now, but their drivers could still use a bit of work and are still sub par (nVidia are still going well now).
Basically my housemate and I have both bought new cards recently, mine being a 6600GT and his being an X1600. For some reason, the drivers for his card wouldn't install whilst a 2nd display device was connected (not just on his machine it seems) and even after installation it just won't display anything on device 2.
Compare that to my 6600GT which installed with no issues and does a nice job in dualview, no hastles.
Of course often your motherboard can push you towards a particular card, for example if you've got an nforce type board, it's not really wise to go buying an ATI card, especially not if you want to Crossfire it with another one, but buying an nVidia card instead will usually work fine.
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