Would this rig be good for gaming/editing hd video ?

You'll want a GPU...

whats a good one for a decent price cause i cant really afford much more and the processor has inbuilt graphics so ?

and the gpu has to come from that website

Depends on your budget but in my mind the best bang for your buck card right now would be a Saphire 7870 - only $359!
http://www.pccasegear.com/index.php?main_page=product_i...

Or a 7850 if you can't find an extra $80
http://www.pccasegear.com/index.php?main_page=product_i...

I'd also change the PSU - get a Silverstone Strider 600W unit and save the cash for the GPU
http://www.pccasegear.com/index.php?main_page=product_i...

thanks for the help