Yes, I believe the WEI works exactly as you suggested. It may be top o the hill for a bit, but soon enough your hardware will look pretty mediocre. Just the nature of the business.
Look at TOMS hierarchy chart here.
As you can see there are a number of cards that are more powerful than the HD 5870, not surprising as it is a generation old.
As for 3dmark, the nice thing about that is you can see how you rate against others with similar specs.