Improve Radeon HD2400 PCI HD video performance

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http://askubuntu.com – I have an ATI Radeon HD2400 PCI (yes, PCI, not PCIe!) in an IBM SFF which I'd like to use as an XBMC media center. I've installed a headless Ubuntu instance from the ubuntumini.iso to which I've only added XBMC (and X.Org of course), so it's not running any window manager other than XBMC, nor any other services other than SSH. RAM usage is unsurprisingly very low (always less than 20% of my 4 gigs) and CPU usage is around 35% (P4 HT 3GHz) when playing an HD video (example is 1280x544 ~5Mbit/s). But while playback is almost ok, the framerate is too low (only 16fps) for HD content to be watchabl (HowTos)