Ahh yes. One of the most argued points in building a system.
What I've learned over the years;
1. GPU should be balanced with the rest of your system.
2. Buy the single most powerful GPU your budget will allow.
3. Brands matter. As with all technology, who makes the GPU makes a difference in the overall customer experience. Nvidia=EVGA/ATI=xFx
4. Research, research, research. For the past few years, ATI has distributed a series of drivers and suites that are exceedingly unreliable. Knowing the most about your products, before you hit the "Confirm Payment" key, will result in a much better experience over the life of the build.