Pretty much every 7850 can do 1050 on stock volts.
So thats the absolute worst OC you could get if the card was utterly utterly terrible.
Most however do around 1150 core with a volt bump. Good ones do 1200+
Just make sure you get a card with a decent cooler on it, don't go for the el cheapo VTX, as you'll end up wishing you'd gone for a card with a better cooler, or end up having to fork out for a 3rd party cooler and ruining your warranty.
Just go for an MSI/Sapphire/HIS from the start.
Not entirely true.
A stock 7850 will match a 570 in a lot of games, in other more nvidia friendly games the 570 will pull ahead (things like BF3).
A 7850 at 1050 core will beat a stock 570 in everything, while using considerably less power, running far cooler, and having more memory.
A 7850 at 1200 core will match/beat an oc'd 570/stock 580 in everything, while having the benefits listed above.
The 7850 is an all round better card.