Output options for the integrated graphics are very impressive with HDMI, DVI, VGA and DisplayPort all supported. All the digital outputs include HDCP support so no matter which display you choose you'll be able to watch protected HD content like Blu-ray discs. The only downside is, although two displays can be used at once, VGA has to be one of the options, which limits your options somewhat.
If you do decide the integrated graphics is not powerful enough, you have two options. Either you add in any old discrete graphics card and turn the integrated graphics off, or you can add-in an ATI HD Radeon 3450 and take advantage of the 780's other big trick, Hybrid Crossfire.
Like regular Crossfire, this uses two graphics chips to split the task of rendering a 3D scene to increase performance. Because of the way Crossfire works, it's only sensible to run Crossfire with similar performing graphics hardware (otherwise the slower chip becomes a bottleneck to the faster one), which is why Hybrid Crossfire is only available with the HD 3450.
There's also no support for a system like nVidia is developing with its Hybrid SLI platform whereby any discrete graphics card can be plugged in and the system will switch between the integrated graphics and discrete card depending on system demand - the advantage of this being you can save power by using the integrated graphics while just doing desktop work but you still have the 3D performance of the discrete card to play games in their full glory. Support for this may be coming in the future but because of the massive driver development involved it's not being enabled yet.
The south bridge, in general terms, controls the communication side of things, so it includes the hard drive, optical drive, and USB controllers, but also handles things like audio.
Although the new SB700 south bridge has been announced along with 780G and 780V, it is actually independent and will be a part of other upcoming chipsets from AMD. It brings with it support for twelve USB ports and six SATA ports (up from eight and four on SB600) and claims improved transfer speeds for both, which sounds good but actually just brings it more in line with what other chipsets can offer.
The same audio controller as before is onboard so you get support for 5.1 channel DVD quality Dolby Digital or DTS and you can use the 780G's HDMI connection for passing this audio signal out to a TV without the need for any additional wires. Unfortunately, this setup can't cope with the 7.1 channel DTS MasterHD or Dolby TrueHD that you can find on Blu-ray or HD DVD discs so you won't be able to get the absolute best sound quality possible. It's not going to be half bad though!