Vista 64 bit or Linux

[UA_Iron]

I think Linux is a pretty involved route unless you go with a more common flavor like Ubuntu

Probably heavy google usage is necessary for this choice, here's a start:
Video Editing For Ubuntu ~ IT Professionals
Lifehacker - How to edit video on a Linux system - Linux

Photo Editing With the GIMP

I like the server 2008 idea too.

I run XP 64 at home and on my workstation at work and its been awesome. Two years ago I would not have said the same thing though.

Go with a WD cavair black series harddrive over the one you have listed, it will give you better performance:
Newegg.com - Western Digital Caviar Black WD7501AALS 750GB 7200 RPM 32MB Cache SATA 3.0Gb/s Hard Drive - Internal Hard Drives
Newegg.com - Western Digital Caviar Black WD1001FALS 1TB 7200 RPM 32MB Cache SATA 3.0Gb/s Hard Drive - Internal Hard Drives

 

[Apokalipse]

There is also Windows XP x64 edition.
I've been using it for a fair while now, and I like it.

 

[Puddle Jumper]

If you do want to move away for windows I recommend looking at PC-BSD. It may not be as popular as linux distro's like ubuntu but it is a great os.

PC-BSD - Home