For your needs, you can even consider the i7 860 and P55 mobos...
They also offer very good performance and the Intel Turbo boost on these chips are better than the 920's...So you wouldn't even need to overclock them...
And one more main reason is the heat...
The i7 860 are 95W TDP CPUs compared to the 130W of the 920, hence less heat and uses less power...
CPU + Mobo
http://www.newegg.com/Product/Comb [...] mbo.276272
RAM
http://www.newegg.com/Product/Prod [...] 6820231276
PSU - Definitely change it...It is not of high quality and never skimp on the PSU...
For a single card, go with atleast 500W PSU from Corsair/ Antec EA/ PCP&S/ Seasonic/ Silverstone...
For Crossfire, get the Corsair 750TX...
For your questions :
1. ^ Given above...
2. If the mobo supports crossfire, then it will have enough space...and as for the case, that M9 can accommodate the HD 5850 but just check that they dont have any issues with the power cable to the graphics card and the HDD cage...
3. Yes that memory is compatible with that board.