Hi purple stank,
I do not have a definite answer as to the risks of an underpowered PSU, but I also am the owner of a no-brand PSU like I told you in the other thread.
My PSU is rated 4/10 on jonnyguru.com and on the review it said it delivers a max of 340W which is enough to power my Phenom II x4 945 (140W) and my Radeon 4850 (150W max), and the rest of the PC which in most cases consume trivial amount of power.
A 6850 consumes less than a 4850, so I am waiting for mine (should arrive today
)
So in your case, unless your PSU is complete crap, it should deliver more than 300~350W..
Now as for your options, you could:
1) Test the graphic card for stability under load. And if it runs fine for a couple of hours, then I doubt you will ever have problems related to the PSU. If it doesn't, then replace PSU.
2) If you are really skeptical about your PSU, and don't want to risk anything, just replace PSU right off the bat... It will cost you some extra $$ now, but if you get a good enough PSU you can crossfire in the future with another 6850 (You could still get a power supply just good enough for your system..but make sure to get a good one and read reviews on it).
Hope this will help! And let us know if you have more questions.