Gigabyte ATI HD4890 crashes at overclocking

Have you tried a lower overclock? It looks to me you exceeded the maximum overclock for your card.

no i havent tried but i tested it with ccc and it says its fine that's the reason why i don't understand

Then your card can't take more than your current speed.
Same as rolli, try to lowering your speed...  

Yup, lower your speed! Have you checked also the temp of your card during OC?
Don't forget also to check it's temp! You don't want to damage your card due to everheating.

i forgot to say it, yes i did, i even tried the fan at 70% or i dont know and nothing changed but i'll try again

I'll lower it too

That is a pretty high OC for that card unless you are increasing the voltage. Try 900Mhz on the GPU. Just because CCC is stable doesn't mean everything will be. I've had stable OC's that worked for months and then a GPU demanding game comes out and kills it.

Download OCCT and run the GPU test and run it for an hour if you REALLY want to test an OC. It will turn your card into a space heater, but will prove out you OC.
http://www.ocbase.com/perestroika_en/index.php?Download

i just tried it again overclocked at 930 and 1030, put the fan to 70%, when i checked it out, it was 65°C. It crashed after maybe 3 minutes, the screen turned gray, i could still hear the sound of the game. I had to restart the computer as usually

okay im trying this jay

it works fine, i played at least 15 minutes and it didnt crash. 900 and 1030, should it try 910 or else ?

I'd leave it at 900. You won't notice 10MHz more, but it might push you over the stability edge.

Leave it at 900 for a while and see where you are at. You can try 910, but OCing is like babysitting a 7 year old with ADHD. You have to keep watching it and even though everything seems fine for a while, it could spaz out anytime. 900 is a nice round number. You can probably get 1050 or more on the memory. I like round numbers to work with, OCD kicks in.

Try OCCT's GPU stress test for an hour, turn on error checking to. 900/1050 seems like a solid OC.

ok thanks