Silverstone is a good brand but that PSU only has 25a or 300w on the +12v rail. It would handle an HD5770/6770 but an HD6850 would be pushing it and you can do better for the money. This Antec unit is cheaper and has 84w more watts on the +12v rail; http://www.newegg.com/Product/Product.aspx?Item=N82E168...
Usually I would recommend something a bit higher for a H6850 but it should do the job if you aren't overclocking and I'm not seeing many good deals on newegg at the moment(the one purple linked is good but out of stock as you say.)
That seems to be an actually decent low end PSU from Coolermaster(the first I've seen.) It does have 2 more amps on the +12v rail so you might as well go for that.
Alternately there usually are deals for PSUs that are better than either of those for around this price on newegg but seems like it's not the case right now. If you can hold off a bit one will likely turn up before too long.
You don't have to wait for that specifically to come back in stock. Newegg has several sales per week and usually there will be a good deal in the $50-60 area. It's actually a bit odd that there isn't one at the moment.
(Actually i guess there is but it happens to be out of stock.)
Although both of these are way overkill for your needs it's always good to have them because you can always bring them onto another build since they won't use anywhere near their max rated wattage will make them last a long time.