Give me a slight guide to overclocking a 7850?

Go up in little steps (10-20MHz steps) and check for stability, once it gets unstable step back down to the highest stable point you can find.

Which tests should I run? And how will I know it is unstable? Think these are the only things that I am struggling with atm.

http://www.youtube.com/watch?v=gA_l5-HDel4&feature=rela... and maybe this http://www.youtube.com/watch?v=QYeCVhGAHHs

If the card you have bought has an factory OC'd version, you can generally jump straight to those settings and go from there. Hardware wise both cards should be the same.

Also in general, a higher core clock = better performance than a higher the memory clock. So if you have an unstable clock, decrease the memory more than the core. I think the ratio is 1Mhz on the core = 4Mhz on the memory in terms of performance boost. Don't hold me to that though.

That NCIX Tech tips Video is very good, I followed that and I am enjoying a fairly good overclock.

Just one caveat, for some reason I didn't get any artifacts or issues running Furmark (or MSI's Kombuster) with a bad overclock.
You will have to play games or use 3D Mark11 to ensure you have a stable overclock.
Though Furmark is good for getting worst case scenario temperatures.

http://www.3dmark.com/3dmark11/download/
http://www.ozone3d.net/benchmarks/fur/