This might be of some help to schools which are looking into getting Raspberry Pi's in their computer rooms but don't want to have to buy new monitors. When you're setting up VGA monitors on the Pi the best thing to do first is look up the monitor specs online and find out the max / native resolution.
In my case the native resolution was 1440x900. See here for mine. You need to Google something similar for your monitor. Usually if you just type the make and model number along with the word "specifications" you'll find what you need.
Once you know the max / native resolution you need to look it up, under group 2 (DMT), in this list, that will then tell you what HDMI mode number you have to use in config.txt.
Then you need to get a good HDMI to VGA adapter.
This is one of the cheapest ones I have found; http://www.amazon.co.uk/dp/B0088K7QUQ
At under £10 that makes it pretty viable to equip a computer room with them and saves the expense of buying new digital monitors.
In my tests that adapter worked pretty well. Although the monitor I was using it with caused some blanking. So I used hdmi group 2 and mode 46 for reduced blanking in the end. I then fiddled with over scan settings to move it around a little bit more to perfect it. There was still a bit of a blank area on the left, but I think that is about as good as you'll get in some cases.
Blanking is when the left hand edge of the picture starts somewhere in the middle with most of the image disappearing off the right hand edge of the screen. If you ever see this try one of the reduced blanking modes.
It's always a good idea to just test that the hdmi to vga adapter is working by setting hdmi_safe=1 in config.txt, then once you've verified that is working move onto trying to get the max / native resolution working. That said, the config.txt settings I used were;
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hdmi_force_hotplug=1 hdmi_ignore_edid=0xa5000080 config_hdmi_boost=4 hdmi_group=2 hdmi_mode=46
Obviously your monitor is probably different to my one so you'll want to use a different hdmi mode number. You may need to play around trying to get the best mode with a refresh rate that your monitor likes. The best thing to do is to SSH into the Pi using Putty to make the changes to config.txt, then reboot and watch how the screen looks.
Also, my monitor has built in speakers which can be connected to the Pi using a 3.5mm headphone jack cable. You can use the following command to make the Pi redirect audio output to the audio jack. Then you should be able to get sound out.
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sudo amixer cset numid=3 1