sweet which card do you run?
1. Don't ever get caught loving or hating either manufacturer too much.
2. Google benchmarks for any card that your looking at. Benchmarks also tend to come with reviews.
3. Google crossfire with your mobo model and see how many problems people are having. Remember, you're probably going to see a lot of them because most people don't post up how well things work - only how much they suck.
In short, doing some googling will get you all the answers you need. Benchmarks are supposed to be a good measurement, but only use them as a guideline and look at the same benchmark comparisons at different places. Why? Because they've been known to be rigged and otherwise not equal.
A lot of time reviews are not 100% what we'd like. No different then the media, people can put their own spin on it.
However, most people only have opinions from their experience and that does not necessarily mean they are correct. Get as much information about the cards you want and make a decision off that.
This post is a good start, but do the googling. Maybe also post up in a couple of other places as well. (More tech forums like TX Redneck frequents and such)