Need help with Building: Video editing + Gaming Rig

Hey everyone,

I need advise from the experts on this fourm on a new system I am building that will be used mostly for HD Video editing + Some gaming.

I will list the parts I'm considering. Please let me know if there are any issues compatibility wise or performance with some of these parts. Also please feel free to recommend something else that you think will be a better optioin.

My Computer proposed build:

1) Case: COOLER MASTER COSMOS 1000
http://www.newegg.com/Product/Product.aspx?Item=N82E168...

2) Processor: Core 2 Quad Q9450 Yorkfield 2.66GHz 12MB L2 Cache

3) MOBO: EVGA 132-CK-NF78-A1 LGA 775 NVIDIA nForce 780i SLI
http://www.newegg.com/Product/Product.aspx?Item=N82E168...

4) Power Supply:
OCZ GameXStream OCZ700GXSSLI 700W http://www.newegg.com/Product/Product.aspx?Item=N82E168...

5) HDD:
SAMSUNG Spinpoint F1 HD753LJ 750GB 7200 RPM 32MB Cache
http://www.newegg.com/Product/Product.aspx?Item=N82E168...

6) Video Card: EVGA GTX 280

7) Memory: Not sure yet. Please recommend.

8) Anything else I left out.

Thanks in advance for your help.

P.S. I'm looking at SAPPHIRE Radeon HD 4870 512MB Video Card vs GTX280.

Could you please recommend MOBO and other parts if I go with the 4870 option. It seems like a better value.