hey guys im just wondering whats better tubing for my 2x 285. i have ek 285 water block and i just bought corsair obidian 800d so room is not a problem. 3/8 or 1/2 that is the question ...lol 

1/2" is pretty much the most popular or 7/16.  I wouldn't use 3/8 myself.

mygtx285,  Not sure if you realized this, but you posted this in the wrong section.  This belongs in the GT200 series section.  I'll move it for you this time, but please review the TOS again.  Link below.  Next time is a warning.

EVGA forum TOS

mygtx285

hey guys im just wondering whats better tubing for my 2x 285. i have ek 285 water block and i just bought corsair obidian 800d so room is not a problem. 3/8 or 1/2 that is the question ...lol 

I would do a little research into fluid mechanics before I bet my processor or graphics cards on what "everybody" knows.
 
A lot of posters, even those with 3000+ posts are still morons at the end of the day and most of what nonsense they spout is total garbage that they just read somewhere and picked up as absolute truth.
 
The tubing itself is indeed important, but the system itself is only as strong as its weakest link. What pump do you have? What radiator are you using? Do you know the heat transfer characteristics of your cooling fluid? You can't just use water...the addition of other substances into the cooling solution to make it less corrosive and less prone to biological contamination change the fluid's characteristics.
 
To be honest though, without pushing overclocking to extremes, you can probalbly get away with using anecdotal evidence and hearsay in your build and will likely be fine. I'd run a fluid simulation in CAD first, but I am a mechanical engineer, I live for this stuff.