As I hear Ati's AVIVO is actually better (and free) compared to Nvidia's Purevideo, so I'd check out Ati cards instead. If you're concerned about Ati cards temperature, then you must remember that they are being built on the newer A12 silicon that is designed to sustain those temperatures, but you can always get a non-reference cooler model (Sapphire, ASUS, HIS, and Powercolor come to mind) that can get those temps really low (that's what I did with my HD 4850.)
I'd go with the Q9550, but get an aftermarket cooler (Xigmatek HDT 1283 with Noctua or Scythe S-Flex fans come to mind) with a silent fan, that way you won't run into any noise issues while playing your favorite movies.
One more thing I'd like to suggest you is that since you are a video enthusiast and I'm sure you want to see full HD videos, those take a lot of space. Given the fact that you are already planning a RAID array, I'd suggest two Western Digital 640GB drives to provide you with all the storage you'll need.
If you do decide to get an Ati card, then I'd suggest a nice P45 motherboard or maybe even an X38 board (X48 isn't worth the money, since it has almost the same capabilities as X38s, but with a $150 premium on them). And 4GB of DDR2 1066 ram.
Links to products I mentioned:
Western Digital Caviar SE16 WD6400AAKS 640GB 7200 RPM 16MB Cache SATA 3.0Gb/s Hard Drive - OEM
http://www.newegg.com/Product/Product.aspx?Item=N82E16822136218
Intel Core 2 Quad Q9550 Yorkfield 2.83GHz 12MB L2 Cache LGA 775 95W Quad-Core Processor - Retail
http://www.newegg.com/Product/Product.aspx?Item=N82E16819115041
XIGMATEK HDT-S1283 120mm Rifle CPU Cooler - Retail
http://www.newegg.com/Product/Product.aspx?Item=N82E16835233003
XIGMATEK ACK-I7751 Retention Bracket - Retail
http://www.newegg.com/Product/Product.aspx?Item=N82E16835233019
Scythe S-FLEX SFF21E 120mm Case Fan - Retail
http://www.newegg.com/Product/Product.aspx?Item=N82E16835185005
POWERCOLOR AX4870 512MD5-PPH Radeon HD 4870 512MB 256-bit GDDR5 PCI Express 2.0 x16 HDCP Ready CrossFire Supported Video Card - Retail
http://www.newegg.com/Product/Product.aspx?Item=N82E16814131117
OR
ASUS EAH4870/HTDI/512M/A Radeon HD 4870 512MB 256-bit GDDR5 PCI Express 2.0 x16 HDCP Ready CrossFire Supported Video Card - Retail
http://www.newegg.com/Product/Product.aspx?Item=N82E16814121276
DFI LANPARTY DK X38-T2R LGA 775 Intel X38 ATX Intel Motherboard - Retail
http://www.newegg.com/Product/Product.aspx?Item=N82E16813136045
OR
GIGABYTE GA-EP45-DS3R LGA 775 Intel P45 ATX Intel Motherboard - Retail
http://www.newegg.com/Product/Product.aspx?Item=N82E16813128344
OCZ Reaper HPC 4GB (2 x 2GB) 240-Pin DDR2 SDRAM DDR2 1066 (PC2 8500) Dual Channel Kit Desktop Memory - Retail
http://www.newegg.com/Product/Product.aspx?Item=N82E16820227289