So many questions...........

ok, so i want to upgrade my pc in a few days and Ive picked/deciding between the following parts:

i7 3770k / i5 3570k
asrock z77 extreme 4
a second 7850 / 7950

the rest of the components will be migrated from my current rig for now, apart from the psu if someone could kindly suggest one capable of safely running a rig like this without a huge price tag . Also what problems might i run in to if i add a second 7850 (my preferred choice) instead of a single 7950?
thanks in advance

3570K is probably your best bet. You using this for gaming? The 3770K has hyperthreading, so unless you run apps that take advantage of the extra cores the money is really not going to benefit you much at all.

7850 vs 7950...I'd have to go look at benchmarks. Normally you want to double up on the higher end cards within a generation. Performance-wise...they're very similar I believe. I am a fan of a single card all things being equal personally, as the power requirements are less as well as more potential. If you can sell the 7850 I would recommend the 7950 and go to the 3GB, which will be beneficial longterm as two 2GB VRAM cards are still limited to only 2GB, and games are beginning to get more memory-heavy on GPU's.

3570K for a gaming rig, yep.

As far as dual cards... well, like atclaeys, Im a fan of a single card for a 1920x1200 or less monitor. 2560x1440+ or multipli monitors is where SLI/Crossfire are really warranted and shine. Sell the 7850 and get a 7950/7970 is my advice there as well...if the 7850 isnt doing it for you...