Ubuntu 14.04, Mint 17; MS Win 8.1.
Will not respond to PM requests for support -- use the forums.
At least xrandr support is quite convenient in setting multiple screen configuration, once you have found the correct set of options, try it to solve the resolution difference on dual monitor.
Not Me, I still have 2 ATI legacy cards in Ubuntu boxes.That's why I'm glad I dropped ATI several years ago
ATI Radeon X550(rv4xx)fanless and X1650(rv5xx).
x300 is a rv/rs X4xx chip and they have never had any hardware acceleration, only software acceleration and that is CPU bound.I still have an x300 card and cannot use any decent acceleration with Ubuntu
Acceleration works decent for me with X550 card, 2GHz duel core CPU, and r300g OSS driver on 10.10 Ubuntu.