A 7850 can barely do 300 MH/S at stock speed. A 7790, at about 60% of the cost of a 7850, can push 300 Mh/s overclocked.
I've found the best bitcoin gpu is a 7950, pound for pound. My Visiontek 7950 reaches 560 Mh/s easily, no overclock necessary, and it runs cooler than my 7850. Now, my 7790 runs much cooler than either the 7850 or the 7950, and it sips power, not like the power hungry 7950.
My 2 cents.
I paid $145 each for the ASUS 7850's (New from NewEgg) and they are both running stock speeds at Intensity 16 hashing scrypt at 340-342KH/s. On Win8 running cgminer 3.10 with AMD 13.4 driver. Tried 12.8, 12.10, 13.1 with three different version of cgminer until I found the best performance. To get a 7950 up to 680KH/s scrypt uses a lot of power and not every card can go that high. These two ASUS 7850's do it at stock speed with voltage at 10.5vdc. Do note here that I am running scrypt hashing and not SHA-256.
As far as heat, they get up to 60C during the day (over 100F outside) and drop down to 53-55C at night. The fans hardly even run on them at that temp. My stock Sapphire 7950's cook up to 78C with their fans at 85-90% during the hottest part of the day. Yesterday I bought a small 9" 120v fan from Walmart and pointed it at the backside of the 7950's ( the fan intake, not the exhaust) and now they are running 72/73C during the day with their fans running at 60%. The 7950's really heat up a room quickly. I have shrunk heat shrink tubing with the 7950 exhaust.
Now, the used XFX Black 7850 that I have is a hot head. It runs that hottest of all of my cards, even with 10.5vdc. So more than likely it needs the heatsinks reset with some better thermal compound. Card runs rock steady it just gets hot doing so. I picked up another 9" fan today to try and cool down the XFX 7850 and now the ASUS 7850 in this machine is scrypt hashing right now at 342KH/s and running at 47C. The XFX dropped down to 73C with the fan. Looks more and more like a thermal compound problem with the heatsinks.
