Crossfire on 2 different cards?

No, you cannot crossfire a 5870 and an 4890. You can only crossfire cards where their families are the same like a 5870 and a 5850 or a 4890 and a 4850, their first two numbers need to match(one exception is the 5970 works with 58xx cards)

Yes, you can't CF those cards, just like hunter said above...

Hey, you already has HD5870, no need to CF, that card is fast enough even without CF.

Overclock the 5870, makes 1GHz core quite frequently.

Yeah i'll oc it instead. Thx for the replies  

Hmm, when i oc'd it to run at 900/1250 clocks in ati overdrive today i noticed that sc 2 get "blackscreens" which means that when i play it freezes and the screen goes black. when it does this i can clearly hear that the gpu fan is turning off and after like 2 secs it goes back on. Also i set the fan manually to run at 65%, it passed ati's test and while i was stress testing it the temp never got over 55 degrees celcius.

I recommend using MSI Afterburner to overclock instead. If your HD5870 is one of the models that allows for it then it will let you raise the voltage of the core for a better overclock. Even if not it is just better with good hardware monitoring which you can even send to an onscreen display during gaming;
http://downloads.guru3d.com/Afterburner-1.6.1_d2562.htm...
To use it with non-MSI cards after installing open MSIAfterburner.cfg(in the program folder) and look for the line that says;
EnableUnofficialOverclocking = 0
and change it to
EnableUnofficialOverclocking = 1

So My problem depends on having too low voltage on my card if i got you right jyjjy. Thx for the software tio and the reply!

gah it says my card doesnt support voltage control. I did set the EnableUnofficialOverclocking =1
UnlockVoltageControl= 1
UnlockVoltageMonitoring= 1

but still cant, tried GPU Tool too but it also said that my card doesnt support voltage control. Any ideas?

The latest MSI Afterburner supports voltage control...

Only if the card has the proper hardware. Most current HD5870s do not let you change the voltage.

Nothing you can do really unless maybe you just bought it and can send it back. This is why you need to do research/ask questions ahead of time. I would actually recommend a nice HD5850 with good cooling that lets you bump up the voltage like the ASUS DirectCU rather than an HD5870;
http://www.legitreviews.com/article/1273/1/
The HD5850 uses the same processor as the HD5870, it just has 10% of its shaders disabled and lower stock speeds. Because they are the same processor they both OC to the same levels and either card can usually get up around 1ghz with a voltage boost. At those speeds the performance difference of the HD5870 is only 5-10% which makes the HD5850 the much better buy considering the price difference.