check your temps, both on the processor and the card through ATI's control panel.
high temps can cause an automatic throttle and your GPU/memory may be running at a fraction of their speed.
A good way to test is to remove automatic clock control for your GPU and memory. Set it to manual as if you're overclocking the card, but don't actually overclock the settings, leave them at stock.
This will force the card to run at top speed in all of your applications. It's power hungry, but it may fix things