Based on personal experience of having to fix them......
Don't buy a Sapphire card
Quality control seems substandard, failure rates are higher than for other ATI makers and theres no UK support site to handle RMAs. The agency who did it lost the contract a couple of years ago - allegedly because they were RMAing too many
Generally ATi based cards are more reliable than nVidia based ones - but Sapphire are the exception
At present I'd always prefer a card with an ATi chip - but not one from Sapphire
As to why I prefer ATi over nVidia - its because nVidia still have problems with low-lead solders fracturing through thermal cycling. Its why you don't see many laptops with nVidia graphics chips fitted