There's a massive thread here on overclocked 7850's
http://forums.overclockers.co.uk/showthread.php?t=18389760&highlight=7850+overlocking
From what I can gather looking at threads an overclocked 1ghz 7850 seems to match a gtx 580 or there abouts
http://www.anandtech.com/bench/Product/517?vs=550 it's kinda always the same so an overclocked 7950 at 1ghz + matches or is just behind a 7970
but an overclocked 7850 i'd say is a bit behind a 7950 at stock it's not as good as the gains on a 7950 to a 7970..(but might have to look in to that as latest 12.11 drivers have added a nice boost but whether or not that brings the overclocked 7850 to match a 7950 at stock I cant tell you but it probably does)
There's always a risk that any card you buy doesn't overclock that great though but most seem to be hitting 1ghz with ease.Seem better off throwing as much as you can at just a card no need to watercool a single card these days especially when you look at the cost but if you plan on going sli/xfire later it would be well worth it.
You also need to be careful choosing a model because I believe some don't allow voltage increases.