While ATI's support has gotten better in the last (1-1/2?) years, Nvidia's support is still much better (for Linux). ATI did a great service to Linux by making a big deal about starting to do video decoding with the video card (mpeg, x.264, etc). While their effort seems to have pretty much stalled since the first of the year, it did put the spurs to Nvidia's VDPAU program. Unfortunately VDPAU will only be supporting series 8 and 9 cards.
Fortunately for you the cpu you have chosen is more than enough to decode most codecs. For 1080p your card MAY be a little short on memory. Hopefully your card has HDMI output as it greatly simplifies doing surround sound. Make sure your TV has at least DVI inputs (preferably HDMI). For anything over 40" make sure the TV is 1080p(not just 1080i).
I would skip the external drive unless you need it for backup purposes.
For just playing videos 2GB will be fine (1GB would probably be fine). On the other hand memory is cheap right now.
The number one thing people building there own machine screw up on is power supplies. These days Seasonic is about the only brand I will buy. I had several machines with other name brand PSU (antec among others) that I could not figure out what was wrong(general goofy behavior). Put in a Seasonic PSU(same wattage) and all the strange issues just went away (permanently).