OK, if you plug the 290 into it, and it draws full power for a few seconds and you are unlucky then the PSU fails, and worst case scenario kills your card, your mobo and your CPU. You might get lucky and it might not do this, it might last 5mins, maybe 10mins, it might fail peacefully, but unknown brands are unknown for a reason.
As to whether it might only be 600W effective, if it was that'd be OK, recently there was a thread on here about a 500W PSU that looked more like a 250W PSU from the rating plate, so it could be as low at 350W and it wouldn't be a surprise.
You choice, i've just told you the risks and the consequences, your call now.