www.tomshardware.com
HotHardware.com - Technology Tested and Burned In
It's not just about the cards specs though, else there'd be no need for all this benchmarking to determine the cards power.. It's mostly to do with the architecture... A card with a 3423789047389257ghz core but the worse architecture in the world would fail against a 700mhz core with good architecture (for example).
Mostly if you want to be able to make informed choices, you have to regularly keep on top of what archtictures offer good results - and then look at the card specs.
Or just look at one of the above sites and see how well the card does in a benchmark. lol