PSU isnt powerful enough for the 7950
and doesnt have enough connectors.
The board you chose is severely limiting
Case is quite crap
Yes it is. Easily.
Molex to 6 pin adapter, about £1.
Only if the OP will be overclocking loads, but I have amended it.
Actually the case is quite good, especially at the price point. It's certainly better made than the Source 210 that you specced.
I've located a copy of Windows 7, so no need for an OS.
Thanks. There may be a possibility of an extra £100 to stretch the budget to £850. How would the extra be better spent? On a better GPU?