All of the card brands are about the same. They have to stick pretty closely to a reference design from AMD/Nvidia. They may have a few small differences in heatsink / cooler etc.
The main difference is warranty, tech support, customer support, etc. The vast majority of cards are manufactured in asia (for ex. ASUS is from taiwan). So some of them have horrendous tech support. While some companies, like Asus, have a strong US presence; and therefore pretty good tech support.
It pretty much all boils down to brand loyalty. And yes, that VisionTek card is OK
The computers are not bad, considering that you have to pay $100 for a copy of Windows 7 OEM. That's the reason they cut budget and custom some outdated components such as the HD 6670 or the HD 6770 video card. If you are up for a build your own pc challenge perhaps you can take a look at this $800 / $900 Pure Gaming PC Build article, just to have a good understanding of what a $800 system can bring. Then price it out again accordingly.