Don't stick to brands. Stick to price vs performance.
Decide how much money you want to spend and then read some reviews/benchmarks which card will give you the most juice at the given price tag. ATI is usually better in the mid-range segment while NVIDIA still holds the edge in the (very expensive) high performance sector.
I have been using NVIDIA cards for years and was happy, then started using ATI and am happy too. Same with processors. =)
+1 Switched from AMD to Intel after a few years with AMD due to the 'i' series of cpus and from ati to nvidia to ati