Both companies have extreme problems with their drivers, but from the situation it looks like Ati is still the worst when it comes to drivers. NVidias for the Geforce 8 series at release were bad, but after some months its pretty decent, however the performance on the new X2900XT changes like +-30% with every new driver release >.<
Heard from others that Ati had those problems with every release, well...
But they have one thing in common: their mid-range cards are utterly crap, the 8600 and the upcoming HD2600 will be so extremely slow, its a joke. Thats why I got myself a 8800gts for 640 for 269€ when there was a discount. Could get nothing for that price when I looked at the Ati cards..
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Intel Core 2 Duo E6420 @ 3GHZ
Gainward 8800gts 640mb
Gigabyte DS3P
4GB DDR-800 RAM
Vista 64bit