Try this instead of the mobo/CPU you have there, or wait for the new Athlon IIs AMD just released to become more available, in which there is a triple core or low power variant. http://www.newegg.com/Product/ComboDealDetails.aspx?ItemList=Combo.273663
You don't need the heatsink, the stock one that comes with the CPU is in most cases sufficient, and I know personally that the one that comes with that specific CPU does its job well and is quite silent as well.
For HDDs, I wouldn't bother with a RAID setup, and just go for one Caviar Black instead - they perform better than the Caviar Blues, and with 500 GB, it's only 3$ more anyhow: http://www.newegg.com/Product/Product.aspx?Item=N82E16822136320
It could just be the codecs/program I'm using, but I've always found that playing back HD videos uses a fuckton of memory, so you might wanna opt for a bit more... memory's dirt cheap anyhow.
I also noticed this: http://www.newegg.com/Product/ComboDealDetails.aspx?ItemList=Combo.277848
Great deal, though the case may be a tad larger than what you want.







