If I had it to do over again, I would get something built locally to my spec with XP (if possible). I have no experience with Windows 7, but once burnt . . .etc. The other alternative I would consider is biting the bullet and going MAC, especially if you are doing video editing.
Windows 7,

from a technical support perspective Windows 7 is really the most annoying, actually I think that the learning curve for a Mac would be shorter if coming from Win XP. A good time perhaps to make the move. Microsoft have done a wonderful job on Win 7 hiding everything useful THEY think their clients use, which THEY think make the OS fall over. Damned irritating.
Yep, this is what I am now doing, I'm building an ultra low powered Win XP machine, using a POV Ion chipset motherboard and dual core Atom 330 processor. Even the 2 Gig of DDR2 memory only set be back about $20. The computer will be silent except for the Samsung Eco 500 Gig hard drive, although I think a 64Gig SSD will be useful as a boot drive. (which I still need to buy). I might forget the 500 Gbyte hard drive though and put my the media files/data on an external NAS as the POV MB has a 1 gig NIC. I'm trying to keep the budget under $400-500. But the PSU is being a bit of a pain to find. I want the computer to be able to run of a 60W PV solar panel. I will even put a T-class PCI audio amplifier board inside as well to run some floor standing speakers so I can watch Blu Ray media in 1080i resolution from just one box - hence the reason it has to be silent. For the power supply I might just have to build a custom PSU, with a big switch on the back to switch from a mains linear Audiophile PSU (using a big lump of toroidal transformer, bridge rectifier, capacitors and voltage regulator etc) and the solar PV/battery array via a
M2-ATX - 160W Intelligent Car PSU to power the PC.
BTW if you are looking for lots of RAID Hard Drive space, a small dedicated NAS setup would probably be preferential (easy to back up using an external HD via USB, without having to turn your PC on and can be easily accessed by even your other laptops using a wireless connection (either by you wireless router or if the NAS drive is wireless capable). Also with a NAS setup, your data is not OS reliant, so even if you upgraded to a Mac, your data i.e. video/data files can be accessed and viewed/ammended etc by MS Windows based laptops, even remotely depending on how good your internet connection is.