I'm not sure about the quality of the HTIB or which one is better, but they would all work for what you want it to do. However, I don't see ARC capability listed on the Denon HTIB. I didn't really examine these choices in detail, but I would be looking at the receiver capabilities and the speaker's wattage & frequency response. Only the Onkyo gives specific info on the specs. The subwoofer definitely doesn't go very deep. The rest of the HTIB don't tell you much of anything.
My suggestion would be to do some research on these HTIBs, check out reviews, ask the same question in the HTIB subforum.
The receivers that come with these HTIB seem to all have plenty of HDMI connections. Connect your tv to the receiver, via the ARC-HDMI connection on it. You can also connect your PC and Gamebox via the Rcv via HDMI. If you are using HDMI to connect your monitor to your PC, I would switch to a another connection or a HMDI to Displayport Cable. That's what I have. DVI doesn't look bad either, but Displayport is pretty close to HDMI.
Anyways, if you hook your PC up to your tv/receiver via the receiver's HDMI connection, that will take care of video and audio. If your tv is not close to your PC, I would suggest going with Mediabridge for a long HDMI connection.
When I say onboard, that is referring to the CPU's graphics/audio in conjunction with the motherboard. Onboard isn't usually referring to a dedicated graphics card. So those that don't have a graphics card, would be using on onboard stuff.
And for the record, if you aren't gaming on your PC, there is little difference between onboard graphics/audio and with a dedicated video card. When I built my computer I first just had the HD3000/realtek that came with my Intel CPU. I connected everything to the motherboard. Then I got a Sapphire HD6850 graphics card. No real difference noticed in playing movies.
My honest opinion would be to build your own HT audio. More bang for the buck and you get to choose what you like. Yeah, this is the easy way to go, but you might be paying more for less sound/quality.