I also have a Biostar 790GX motherboard (TA790GX-128M) and here was my experience (using a 4870):
- Not necessary to disable onboard video. I eventually did just to make sure it wasn't using any extra system resources and to possibly slightly reduce power consumption, but my initial boot was fine without doing anything to the BIOS.
- There are two PCIe slots. Most motherboards have the primary slot up next to the ports. On this one, the PCIe slot next to the ports is the secondary slot, and the one farther down is the primary. I've only ever used the primary slot.
- I did install the paddle card (a small blue thing about the same length as a credit card but half the width) in the other PCIe slot eventually, but I didn't do it right away. It's not strictly necessary to install it, it just increases the PCIe speed from x8 to x16, which rarely has any effect on performance.
If you're getting no video despite having your card plugged into the proper slot, then maybe your video card is bad. You may have to make do with the onboard video for now. If you disabled it in the BIOS, you'll have to reset your BIOS and then you ought to be able to get it back.