Moved to a HD display - and now SD looks like jpgs

[brainsaw]

I've just got a HD monitor and everything is now lovely in HD - but in SD it shows up the compression marks like fuzzyness around text that you get like when you've saved a screenshot in JPG instead of PNG.

Is this just due to the source signal, or can this be improved by a better graphics card? (the one I have is a bit old)