Just ran into this thread from a google search.
I can totally agree with Yuka on this one, and from recent practical experience.
I just changed the two Sapphire HD3870 OC Editions that I had in my system
for one Club3D HD4870. Standard clocks: GPU 750MHz RAM 900MHz.
Immediately, it was obvious that the default fan profile is rubbish. The card
was reporting over 70C at idle! Once I had reprogrammed the BIOS with a new
profile and managed to burn the new BIOS without bricking the card, I had 52C
idle and less than 63C through a run of 3DMark06!
So then I got to thinking 'I wonder how this thing overclocks?'.
I ran the standard 'Auto-tune' in Catalyst Control Center and got:
GPU 780Mhz RAM Clock 1090MHz
With the same fan profile, the card runs two degrees warmer at both idle and
load!! A longer 'Very High' session in Crysis proved the card to be perfectly stable
at this speed.
Here come the big 3DMark scores! Or so I thought. Even though the card handles
well above the memory levels of the 'OC' editions, and nearly the same GPU clock,
my overclock adds about 500 points to my 3DMark06 score... (17234)
So, to the original poster; save your money and stick with the stock card!