Since DVI is a digital signal, it's very obvious when the signal isn't getting through since you get picture breakup, not something minor like ghost images. Expensive DVI/HDMI cables are a waste of money IMO unless you need to use very long cables.
As for VGA, the cable supplied with your monitor should be able to handle the max res of the monitor TBH. Last 3 LCD monitors I've owned I literally couldn't tell the difference between the picture through VGA and DVI, so that tells you those VGA cables were working perfectly. I dunno if maybe with the real bargain basement budget monitors they give you cheap leads, but I've never had a problem with the supplied VGA cable with any of the monitors I've owned. Probably a lot of people forget that you need to use the monitors auto-adjust when using an analogue signal, or they run their screens out of the native res, or they use their old VGA cable with their new monitor to save the effort of unplugging a lead.
It's absolutely true that you can get ghost images with poor VGA cables though. Ideally VGA cables shouldn't run parallel to power leads either since that can lead to interference sometimes.