Electrically computers are a very noisy environment, which makes it difficult to get a good analog signal out of. I haven't managed to find any measurements for it [which is unfortunately almost always true of PC equipment]. It is possible that the setup you're looking at will work out well, I am mostly going off personal bias here.
This is a myth not born out by the fact that there have been many quality internal sound cards where the output is pristine. I run the EMU 1212M PCIe and it's quiet and dynamic as can be.
External solutions are the popular choice for various reasons including easier access to cabling, portability, the ability to design to Class 1 / 2 USB audio spec (driverless) etc...
The HDAV card will be the one the OP needs to use since it can output to the analog for use on amps and still send the video to HDMI.