Overclocking Sapphire 4870X2

I have a month since my system is as follows:

I7 920@3.3Ghz
Trip channel OCZ gold @ 1600Mhz
Sapphire HD 4870X2
Antec 850W power supply
P6T-Deluxe motherboard

Yesterday I thought to go see if my card could Overclocking .I started to calm his tune a car to run with him on a CCC 790Mhz and 990Mhz continued.
This I found in 3DMark 06, a difference of 500 points (too little, right?)

Now, the system quite unstable respond after an hour or something to that clocks. System first exhibits artifacts, then black screen + freeze.

Does anyone know where this could be? Or what I'm doing wrong with?
I done with ati tray tools (CCC, the car Overclock and I used those values thus ATT)

The 4870x2 is anyway no real Overclockers, performance in did small profit, but expects there not too many. Ingame you more than a few FPS out presses. The 4870X2 is from the factory already at max.

(Did you know the fan settings manually inflated?)

What are your scores? If I may ask?

When I run 2 4870X2 cards in CrossfireX Sapphire With the Core at 800MHz and memory at 975. I have the fan running at 80% here ....

This should really up your card!

I have the fan running at 57% as I clocks manageable (temp is not above 75, so that's good)
If it is normal that only a few extra fps out of it but I still consider to return to stock it.
Score in 3dmark 06 was 21,900 with 21,400 without Overclock.

maybe a stupid question indeed, but with a 3d mark score, it is not necessary to overclocking your GPU, you are having low fps games then? 1920x1200 because my system scores over 20000, but I can smile at all games on max 1920x1200

Nope it is not necessary to Overclocken.
It is nice to the maximum out of your system to get where you have paid for it.

All the games I play idd also extremely good at 1920x1080 (NML game on a TV) but that little bit extra from your card presses is always fun.