there is nothing wrong with the setup, but if you're only using it for downloading and watching films, then you may be specing yourself a little highly.
a E5200 would be easily enough for you. (£66)
£100+ for a 8800GT is too much. As you are planning to watch films on a big TV and I'm assuming you will be wanting to use HDMI, I would suggest a ATI card so you can have surround sound over HDMI. nVidia doesn't offer this.
Have a look at something like a HD4670. (£65+)
Get a bigger hard drive. I've got the one you picked out and it's great, but if you're downloading then I would look at 250GB+
I don't know about the K45, but can't see why it wouldn't be fine for what you are doing. Saves you some mor eand it looks better.
You can either walk away with the savings or spend a little more on the keyboard and get a diNovo mini. (£80) costs a lot, but lovely.