Sorry for the double post, but I think I deserves a separate post.
After having a good chat with killari - he pointed out that a cheap, low power AM3 CPU and a 785 motherboard (with integrated HD 4200 graphics) would make a faster and cheaper system than a Zotac 9300 s775 one. Here is a complete spec based around this:
The total cost is a bit above the price of the initial spec by Ryan, but I also included a very quiet CPU cooler and a pair of brilliant fans. The drawback of that the system is bigger, but if you can find a small, cheap Micro ATX case then please chime in. However, one of the many benefits is that it will be extremely quiet and handle any video content you throw at it with aplomb.
Edit: After a look around I have found many Micro ATX cases. However, they don't appear to be stocked on OCUK - so I won't elaborate too much. Suffice to say that they are very cheap, reasonably small (compared to the CM Elite above) and include PSUs that will happily handle the system. In this case the CPU cooler and OCZ PSU are surplus to requirements, though quiet case fans would be a good investment. Depending on the case, a slimline DVD drive may be required.