Could a modxstream pro 500w?

Yes, I think you should be good. Two rails make 36A.

but just one rail of 18 amp handle the videocard, the hdd, etc. so i think it´s not 36 amp in this case

Your card will take two PCIe power plugs, EACH with 18A and each on its own rail, so you'll be fine.
http://www.overclockers.com/forums/showthread.php?t=548...
Read carefully. It really spells it out farther down: "The bottom line is, for 99% of the folks out there single vs. multiple +12V rails is a NON ISSUE. It's something that has been hyped up by marketing folks on BOTH SIDES of the fence."

I think it would handle a 6950.
http://www.hardwaresecrets.com/fullimage.php?image=2445...
http://www.guru3d.com/article/radeon-6950-6970-review/1...

The thing is that i don´t want to overload a rail, because like it explains here:
"Typical multiple +12V rail configurations:
2 x 12V rails
Original ATX12V specification's division of +12V rails.
One rail to the CPU, one rail to everything else.
VERY old school since it's very likely that "everything else" may include a graphics card that requires a PCIe connector.
Typically only seen on PSU's < 600W."

Can i run "everything else" and a video card like the hd 6950 with that rail of 18amp? I may sound like flying around in the same thing too much but I want to be pretty sure about this,  

One rail to the cpu and motherboard, one to everything else, you said so yourself in the OP.
The card will draw power from the slot on the motherboard, hmm power from both rails what a concept.

The card draws power from the mobo slot (~75W) and from both PCI cables (~125W), which should each come from a different rail. What a concept?

Dual rail psu.
1 rail to the native cables, 1 rail to the modular cables.
Both pcie cables are modular.

well, i´m feeling more secure now, i was a little worried about overloading a rail.  

I've been running an HiS 6950 with that PSU for 2 months without any issues. Other components:
1 SSD, 1 HDD, 1 DVD burner, 2 USB devices, MSI 870 G45, AMD Phenom II 720 BE, Corsair H80, 3 case fans.

YMMV but no issues for me.

that´s what i wanted, real facts, thanks.  

Best answer selected by pit_1209.