Sometimes the install detection is wrong. Then you get a blank screen when installed. That is why I suggested another install to see if it detects it correctly for you this time. I would ensure I disabled the onboard video if using an external video card.
If you do get another blank video then boot into the live cd as you are now and research your video chipset and how to fix video errors for Wheezy Linux distribution.
If you do not have any configuration or time involved in your current setup it literally takes 30 mins or so, well worth a try in my opinion.
You can see what drivers are in use in the live cd by issuing this command in a terminal:
lsmod
Among many others which you can research while trying to figure it out. Perhaps another here with that chipset can help you troubleshoot it.