hd6870 - an oldish model card now but still very powerful - it will blitz the 7700 series of cards completely by around 30% in performance - expect high/ultra settings in a lot of games at res of upto 1080p with your i3
bang for buck I still think the best card around at the minute £110
the psu is a very very good buy & will power any card around with that many amps on the 2x12v rails
It has 2x pcie power connector which the 6850/6870 requires - its a very power hungry card - Id definitely buy at that price - you will be needing one no matter what card you buy.
to put in perspective a hd6850 is the equivalent performance wise of the newer hd7770 (discount the hd7750 completely its underpowered compared to either of these 2) but requires around 25% more power
out of 2 choices wth similar performance just price up a 6850 & a 7770 ,so long as both have 1gb of ddr5 memory performance is going to be very very similar - pick the cheaper of the 2 would be my recommendation
the 6870 is 15-20% more powerful than either a 7770 or a 6850,the one I suggested is the fastest one around ,with a 15% factory overclock - I understand that having to buy a new psu puts it out of your price range a little.
this one for £7 more than the 6850 is worth the money imo
£130 inclusive for a card with that kind of power & a new cent qual psu is not to be sniffed at,youll have a very good gaming machine paired up with your i3 - id be expecting 40fps+ on most games at 1080p ultra settings once installed ;-)