Dont download no codec. Unistall all of them and use VLC Player.
I been running test on 2 of my machines before i build a media center this Christmas. Specs
PC 1.
AMD64 3200+ 2.0GHZ
2048MB corsair TwinX
ATi X1800XT
PC2.
AMD FX-60 2.94GHZ
2048MB Corsair TwinX Pro 3500LL
ATi X1900XTX
I played Quicktime trailers in 1080p Using Quicktime on PC1 it was a goner played it very slowly, same trailer played in VLC Player same quality played it better using less memory and cpu. I had task man running during the run.
Same trailer on PC2 Ran perfect i changed Task man to show it as one core and it was using average about 60% in Quicktime no frames missed at all trailer ran perfect. Same trailer in VLC used about 15-20% less CPU and abit less memory and quality was the same.
The AMD 5000+ is a FX60 at 2.6ghz 1mb cache and it will run perfect and are good overclockers if you need the extra performance that chip prob could run 1440p
My specs for Media Cent
AMD X2 4600+ and OC it if i need more from it
2048mb Corsair TwinX
Ati X1600 Pro HDMI 256DDR3
80gb SATA II 7200rpm
10/100/1000mbps network card if one aint on motherboard
Have it connected to my 1gbps switch
Your build is very good i would change the card to somthing with abit more power though. I did see somewhere benchmarks on video playback and how that affect the CPU usage. I think it was in PC Format mag.