I am using an Asus Silent 8600Gt with no problems. It setup the latest "nv" driver when I did the dvd install of 710 (make sure your internet connection is working when installing from the DVD). Then I installed the "Nvidia" prop driver from the Nvidia website and I have had no problems since. Most issues you will find is not with the card but with the Xorg.conf configuration file and that will in most cases work at default but possibly with Overscan or Underscan issues this can be adjusted via modelines in certain cases. MYTHTV has a great db a some tv specific modelines. There is lots of info in the wiki and on this forum for such as well as the Ubuntu and MYTH TV site. Oh Sorry! I am running DVI to HDMI at 1080p and I am playing back all HD with no problem. There are many users I think using slower GPU's with no issues as well. IE 7300, 7600 etc..
Most Mainboards will not allow you to disable the on-board video but automagically puts your installed PCI-E card in place and you need to tell the bios which card gets priority upon booting. It should state in the manual for your board what to do if you are running another video card.
The HDMI board is a good board I know my local pc parts supplier is selling tons of them. The other board I am reading about is the abit. here is a link to the wiki mainboard section. http://wiki.linuxmce.org/index.php/Category:Mainboards
You do not have to run a Matx board unless your case restricts you. you already have the video card now and most of the on-board sound cards can cause some grief as well, so read through the list and see what your options are. I am using a P5K3 Deluxe WI-FI board and it has been up for almost a month straight with no issues. It is a bit overkill as I do not use the wi-fi or the sound but it is 100% rock solid - "touch wood" so far.