The simple way is to just "run" the game yourself.
If you want to make sure you will get all the eye candy you can get from the game, you can just look for reviews on it. If it's an obscure game, you could try looking for specific information in forums of the game and such that can give you an indication on what sort of machine it runs maxed out and compare it to your own.
There is no general rule to it, specially since for 99% of PC games you can play with the detail level and see what works best for a fluid experience.
Plus, you also depend (to a lesser extent) on your CPU. What do you have? As a general rule, any i3, i5 or i7 CPUs from Intel can cope with most games out there, giving smooth framerates across the board. But the rule of thumb is valid: faster CPU, better FPS.
Cheers!
EDIT: Forgot about the Crossfire thingy. That could a tad more painful, since you will need to either wait for the profiles from AMD or create your own (less optimized) profile for the game. It would impact your effective smoothness, yes, but you would still manage to get playable framerates (and hopefully smoothness) tweaking inside the Driver and the game settings.