I would say, that in Crysis , U get @ least 8-10 fps more compared 1GHZ vs Stock.
This is a fact, I wish I still had the screenies, they are somewhere on TPU,
I ran a bunch of tests on Crysis 64bit DX10 with a sapphire 3870 at stock speeds, and also at 875, 950, & 1Ghz.
I have 2 hard-mod 3870 sapphire toxics that have been great to me
A couple months back I was able to get one card to 1147mhz.
The difference in crysis was definitly noticable, all tests were run at 1920x1200, and as the speeds went up so did the frames. I cant remember exactly, but there was a difference of 8 frames from a 1ghz to stock. My best and most consistent results actually came from 950, I got 15 more FPS on average, and it was the STABLE as a rock. the memory on the card plays a huge role in frames at 1920x1200 also. The highest I could bump the memory was 1256, but it made a difference of 6 frames itself
Mods are cool, but since I stopped benching about 2 months ago I have not even touched the voltage regulators I have on the cards. I probably will never do it again