i just bought ati 5870 and i thought it would be loads better than my nvidia GTS250. but when i put in the card i automatically saw a drastic drop in performance. it has the latest driver, but when i play L4D i get 30-40 fps when i had my GTS250 i got 60fps and above. not only are the frames slower but the picture quality isnt as good as my old GTS250. i dont think its bottle necking. here are my pc specs
asus 790fx M4A79T deluxe
AMD Phenom II 955 @3.6ghz
DDR3 4gb @1600mhz
It has nothing to do with a break in period. I have heard of some people having the PCIe being read at x1 or x4 instead of x16 like it should. Go to the bios and make sure any integrated card is turned off and the PCIe x16 is on.
You might run GPU-Z to gather info on it's current state.